Remember the scene where Kirk and Spock got into one of those classic cars. Kirk had trouble with the clutch. I can't imagine that Kirk and Spock travelled very far with Kirk's lousy driving skills, yet they got from one boss' territory to another. A few blocks at most, I would guess.The impression I received is that both Bela Okmyx and Jojo Krako were located in different parts of the same city. Oxmyx might have had the largest territory in the largest city (equivalent to New York?), but more likely it was a territory like that of one of New York's five families in the '50s and '60's, and not a sizable percentage of the planet's surface.
So, you're probably right, the entire mob territory must have been contained in one city. You can't get far in those cars, like from one side of the planet to another, to make a hit.