Is there difference between MC for power vs manual?

General differences:

1...BLEEDING........I'm gonna put that as a "difference" because so many seem to have trouble and it DOES have all to do with hard pedal
2...Rear drum adjustment/ to big rear drums turned out.......Try adjusting the rear drums up "hard" to lock the wheels and see if that fixes the soft pedal

3.....Push rod hole. The recess in the piston for the pusrod is different between manual and power

4....Residual pressure valves.......these are little rubber valves with springs in the ports of the master. Drum brakes have them, disk will have them in the front port for the rear drums. A disk "only" master likely will not. Google/ consult a shop manual On drums these maintain a small positive pressure and help prevent the cylinder cups from intaking AIR

5...Piston size. This depends on how much movement there is in the front pistons, how much the hoses expand, in other words "how much volume." Larger means the pedal moves less far, but you have less effective braking for a given foot pressure. On my 67, I have Linc Versailles rear disk, and Duster/ Demon front. I just "tried it." Defeated the residual valves in the stock master, and it gives me a rock hard pedal at about 1/3--1/2 pedal, and I could lock them up at 70, no problem. My legs are not very strong, anymore