Correct, discs run at higher pressure and a drum-brake booster isn't sufficient. Reportedly it will deceptively work for normal driving, but be inadequate for a hard stop (which is believable).

Which booster to use depends on the car. For instance, the large-diameter flat booster that I have seen on 71-73 Cs will interfere with column-shift linkage on a 68 (it will prevent the linkage from hitting 1st). My 2 conversions have been manual disc and PB but 4-speed, so I have no further info.

Also, anyone reading & planning should be aware that, unlike other Mopars, in a C-body the pedal and bracketry are different between PB and MB. Pedal setup must be correct!