A POWER DRUM booster will not work with disc brakes!
We just went through this on a 68 Charger R/T. We put all of the disc brake parts on the car except for the original Power drum booster that was originally on the car. We had a hard pedal but the car was not stopping well. Changed to a Power Disk booster and stops great now.

Brad
Dorn Restorations

Check out the Dorn's 69 Barracuda "Switchblade" in the Nov.2010 MCG
Check out the Dorn's refurbished 36 Ford in the Feb.2011 Street Rodder