@Albertese - Thanks

Actually I haven't completely given up on Model 1 and as a WIP there will be plenty of changes still coming up to flesh out the blocks I had put in as placeholders. Interestingly, when you start putting in the hallways and such into the TOS Enterprise, it starts to leave proportionally the same amount of engine space as the TMP Enterprise has.
@DakotaSmith - Thank you for well thought out suggestions - and there will be no nasty remarks but a long winded reply
I'll just clarify my intentions regarding this particular project:
"What would the Thermians do?" There are actually quite a few good projects out there that are trying to do what
Matt Jefferies intended or what
Franz Joseph intended but this one is based on the TOS episodes and most of the TOS movies.
The short reply:
For now this is how I am understanding the setup:
The TOS Enterprise is mainly powered by dilithium crystals that are re-charged by it's matter/antimatter reactor system containing 3 reactor cores. There is a reactor core in each nacelle and one in the engineering hull. Even though the nacelles can generate energy it can't be used to directly power the warp engines without going through the dilithium crystals in the energisers first. All the m/am energy is channeled through the dilithium crystals in the main energiser and then made available for the warp engines, weapons, etc.
It can be explained that all three reactor cores aren't involved in every episode. In "That Which Survives", Losira could've just sabotaged the engineering hull core and that'd be enough. In "The Savage Curtain", the aliens could've just disabled or targeted only the warp cores in the nacelles.
As to the vertical and horizontal intermix parts in the TOS ship: it'll probably change and go through revisions. The system does need some kind of "integrator" that brings together the matter and antimatter fuel in the engineering hull and a vertical setup seems logical for grabbing the matter fuel (that could be used for the impulse engines) from up top. There would be power pipes/intermix tubes carrying power from the nacelle cores to the energisers and back up to the nacelles to power the warp engines. I'd imagine more metal shielding and baffle plates than what we'd see on the TMP Enterprise which I think could use a similar setup
And my notes (the long part)
For example, there is plenty of evidence of a multiple matter-antimatter reactor model that indicates that there is a matter/antimatter reactor and fuel in each nacelle:
"The Doomsday Machine" - 35
WASHBURN: We made a complete check on structural and control damage, sir. As far as we can tell, something crashed through the deflectors and knocked out the generators. Somehow the antimatter in the warp drive pods has been deactivated.
Note: Evidence of antimatter in the warp drive pods aka nacelles.
SPOCK: I would say none, Captain. The energy generated by our power nacelles seems to attracts it. I doubt we could manoeuvre close enough without drawing a direct attack upon ourselves.
Note: Since the nacelles generate energy then it's a good chance they have a m/am reactor in them.
"The Apple" - 38
SCOTT: Scott, sir. Our antimatter pods are completely inert.
Note: Could the "antimatter pods" = the nacelles? Or is this specific to antimatter pod containing the antimatter fuel?
KIRK: Then use your imagination. Tie every ounce of power the ship has into the impulse engines. Discard the warp drive nacelles if you have to, and crack out of there with the main section, but get that ship out of there!
Note: Nothing specific to M/AM generation here other than the warp drive nacelles are weighing down the ship.
"Bread and Circuses" - 43
SPOCK: Portions of the antimatter nacelles, personal belongings. Captain, no signs of bodies whatsoever.
Note: More antimatter in the nacelles evidence.
"By Any Other Name" - 50
SPOCK: There is one other possibility, Mister Scott. The final decision, of course, must be the captain's, but I believe we must have it ready for him. The Enterprise is propelled by matter-anti-matter reactors. The barrier we must traverse is negative energy.
Note: An interesting clue. Multiple matter-antimatter reactors. Perhaps the three reactors spoken of in "Catspaw" (two for each nacelles and one for the engineering hull) or just two (one for each nacelle)?
SCOTT: I have opened the control valves to the matter-anti-matter nacelles. On your signal, I will flood them with positive energy.
Note: "Matter-antimatter nacelles" - Yes
But what is this? Flood the nacelles with positive energy?
"The Savage Curtain" - 77
SCOTT: I can't explain it, sir, but the matter and antimatter are in red zone proximity.
KIRK: What caused that?
SCOTT: There's no knowing and there's no stopping it either. The shielding is breaking down. I estimate four hours before it goes completely. Four hours before the ship blows up.
...
KIRK: Scotty, inform Starfleet Command. Disengage nacelles, Jettison if possible. Mister Spock, assist them. Advise and analyse. Scotty? Scotty?
Note: Further evidence that there is matter and antimatter (and it happens to include the fuel) up in the nacelles. And this occurs very late in the production too... episode 77 no less.
"Errand of Mercy" - 27
SPOCK: Minor, Captain. We were most fortunate. Blast damage in decks ten and eleven, minor buckling in the antimatter pods, casualties very light.
Note: "Antimatter pods" could mean the nacelles or just the pods in the nacelles or antimatter pods located somewhere else...
"Metamorphosis" - 31
SPOCK: Helm does not answer, Captain.
KIRK: Neither do the pods. Communications are dead. Building overload. Cut all power relays.
...
SULU: Steady. No, Mister Scott, bearing three ten mark thirty five just cleared. No antimatter residue.
SCOTT: All scanners, spherical sweep. Range, maximum. They'll have to pick it up.
Note: This is shuttlecraft-related but this also hints that those were antimatter pods and thus a warp powered shuttle.
"I, Mudd" - 41
NORMAN: I am in total control of your ship. I have connected the matter-antimatter pods to the main navigational bank. A trigger relay is now in operation. Any attempts to alter course will result in immediate destruction of this vessel.
Note: The pods word again 
but then we also have evidence for a single matter/antimatter reactor
"That Which Survives" - 69
SCOTT: Watkins, check the bypass valve on the matter/antimatter reaction chamber. Make sure it's not overheating.
WATKINS: But, Mister Scott, the board shows correct.
SCOTT: I didn't ask you to check the board, lad.
Note: Scotty indicates only one m/am reaction chamber (not multiples).
LOSIRA: Show me this unit. I wish to learn.
WATKINS: This is the matter-antimatter integrator control. That's the cut off switch.
LOSIRA: Not correct. That is the emergency overload bypass, which engages almost instantaneously. A wise precaution, considering it takes the antimatter longer to explode once the magnetic flow fails. I am for you, Mister Watkins.
WATKINS: Mister Scott, there's a strange woman who knows the entire plan of the Enterprise.
Note: A single overload bypass. Not multiple (at least two) if the m/am reactors were in the nacelles.
SCOTT: Aye, Mister Spock, and I found out why. The emergency bypass control of the matter-antimatter integrator is fused.
SCOTT: It's completely useless. The engines are running wild. There's no way to get at them. We should reach maximum overload in about fifteen minutes.
Note: Interestingly, Scotty couldn't just disengage the dilithium circuits/energisers and just cut the power to the warp engines. Perhaps if he did, then all that m/am energy has no where to go and well... boom!
SPOCK: As I recall the pattern of our fuel flow, there is an access tube leading to the matter-antimatter reaction chamber.
SCOTT: There's a service crawlway, but it's not meant to be used while the integrator operates.
SPOCK: Still, it is there, and it might be possible to shut off the fuel at that point.
SCOTT: What with? Bare hands?
SPOCK: A magnetic probe.
SCOTT: Any matter that comes in contact with antimatter triggers the explosion. And I'm not even sure a man can live in the crawlway in the energy stream of the magnetic field that bottles up the antimatter.
Note: Only one m/am reaction chamber mentioned here. And there appears to be only one place to shut off the antimatter fuel flow to it which doesn't match up with two nacelles with matter/antimatter reactors and their own antimatter fuel. Scotty would need to make a clone of himself if he were up in two different nacelles. Also, the service crawlway room wouldn't fit in the nacelle with the hallway outside.
and then some wiggle room to consider the possibility that the matter/antimatter reactor system consists of 3 reactor cores or chambers (1 in each nacelle and 1 in the engineering hull) that can be turned used individually or all together:
"Catspaw" - 30
DESALLE: Keep it up, Mister Chekov. Channel the entire output of reactors one, two, and three into the relay stations. Whatever it is, it's starting to weaken.
Note: Could indicate three matter/antimatter reactors.
"Galileo Seven" - 14
SCOTT: I can adjust the main reactor to function with a substitute fuel supply.
Note: This could mimic a 3 reactor setup for the Enterprise - a "main reactor" in the hull and one in each nacelle.
"Elaan of Troyius" - 57
SCOTT: The anti-matter pods are rigged to blow up the moment we go into warp drive.
...
SCOTT: I've got bad news, Captain. The entire dilithium crystal converter assembly is fused. No chance of repair.
SCOTT: It's completely unusable.
KIRK: No chance of restoring warp drive?
SCOTT: Not without dilithium crystals. We can't even generate enough power to fire our weapons.
...
SCOTT: Our shields will hold for a few passes, but without the matter-antimatter reactor, we've no chance. Captain, can you not call Starfleet on this emergency?
...
SPOCK: Captain, these are crude crystals. There is no way to judge what the unusual shapes will do to the energy flow.
...
SCOTT: Aye, that could blow us up just as effectively as...
Note: Scott specifies only ONE M/AM reactor but multiple antimatter pods. It is possible that the matter/antimatter reactor could contain multiple reactors (thus the multiple pods). Also with the dilithium sabotaged the m/am nacelles are unable to function indicating that some part of the energiser/dilithium machinery is up in the nacelles and also that generating a warp field or even power for weapons cannot be done with just the m/am reactor(s) but requires the dilithium crystals. Presumably the crystals need to be charged up.
I'll use that to segue into one of the apparent functions of the dilithium crystals in the series - they were super capacitors (or batteries):
"The Doomsday Machine" - 35
PALMER: Sir, Deck seven reports power failure in main energisers. Implementing emergency procedures. Severe casualties reported on decks three and four. Damage control party sealing off inner hull rupture.
SPOCK: It has ceased fire. We're being held in a tractor beam. We're being pulled inside, Commodore. You must veer off.
DECKER: Maintain phaser fire, helmsman.
SPOCK: We have lost warp power. If we don't break the tractor beam within sixty seconds, we never will.
Note: Warp power went when the main energisers failed and the dilithium crystals are tied to the energisers in "The Alternative Factor".
"The Alternative Factor" - 20
MASTERS: Whatever that phenomenon was, it drained almost all of our crystals completely. It could mean trouble.
KIRK: You have a talent for understatement, Lieutenant. Without full crystal power, our orbit will begin to decay in ten hours. Re-amplify immediately.
...
KIRK: Out of the question. Those crystals are the very heart of the power of my ship.
...
Captain's log, stardate 3088.7. We are no closer to finding an answer to the strange phenomenon than we were at the beginning. Not only have two of my crewmen been attacked, two of our dilithium crystals are missing, and without them the Enterprise cannot operate at full power. They must be found.
Note: Interestingly, not antimatter power exactly but "crystal power". This would suggest that the dilithium crystals on the Enterprise are like super capacitors or batteries that are charged up/amplified by presumably the matter/antimatter reactor. It is the dilithium energy that makes the warp propulsion go. The energisers not only provide power to the ship's systems but also act as a recharging mechanism as well. Full power means all four crystals (charged up.)
LAZARUS: That's very bad, Captain. If he comes through at a time of his own choosing. But I think if we hurry and you will help me, he can yet still be stopped. There's little time left. He meant to come through. When you accidentally passed through, it drained his crystals. It'll take him about ten minutes to re-energise with the equipment aboard his ship. That should give us enough time.
Note: Lazarus uses the crystals also as a super capacitor.
"The Paradise Syndrome" - 58
SPOCK: All engines stop. Hold position here.
SCOTT: All engines stop, sir.
SPOCK: Prepare to activate deflectors.
SULU: Aye, sir.
CHEKOV: Power dropping, sir.
SPOCK: Engineering, maintain full power. Full power.
SCOTT: Dilithium crystal circuit's failing, sir. We'll have to replace it.
Note: Scotty has ejected one of the crystals which explains why the ship isn't at full power.
SPOCK: Not now.
CHEKOV: Zero. Deflection point now, sir.
SPOCK: Activate deflectors.
CHEKOV: Power dropping, sir.
SPOCK: Degree of deflection, Mister Sulu.
SULU: Not enough, Mister Spock. It's only point zero zero one three degrees.
SPOCK: Recircuit power to engines. Maximum speed heading three seven mark zero one zero.
Note: Evidence that the warp engines get a power feed from presumably the dilithium circuits/energisers.
"The Day of the Dove" - 66
SCOTT: The ship's dilithium crystals are deteriorating. We can't stop the process.
KIRK: Time factor?
SCOTT: In twelve minutes, we'll be totally without engine power.
...
SCOTT: There's no change, Captain. The dilithium crystals are discharging.
Note: Further evidence that the crystals store power.
A more detailed WIP of the power distribution onboard the TOS Enterprise.