Two questions I just realized I couldn't answer about a 67 Coupe SB 4spd:

A. Other than factory correct issues, what's the difference between the function of a 7 or 9 or 11 inch booster? Other than a smaller size to allow for engine clearance, does it matter which one you use?

B. Can you just add a booster to a manual drum brake system and make it a power drum system? Are there other parts you have to change? (not counting the pedal rod)

not sure about A.
but I have installed the boosters to manual drums a couple of times and never had any issues
not really sure if disc booster is the same as drum booster I think I still have a booster left that came from a drum setup
I will check the part numbers and get back

Disk brakes require more vacuum. You can put a larger booster on if you have clearance on your inner fender or use a dual diaphragm booster (ford) which has a smaller diameter.

Do different size boosters have different vacuum requirements?

I am currently having vacuum problems with my 396.

Tim

Yes: I would say different size boosters operates at different vacum levels therefore, delivers more or less output pressure 600 to 800 PSI for passenger cars.

So are we saying an 11 inch booster is better than a 9 inch for discs?