Brake pedal is really stiff

I don’t think it is a master cylinder diameter problem. I have manual K&H disks on the Dart and same general sized diameter MS, and the brakes work & feel normal. My pedal is solid and firm once friction material contacts metal.

Granted on car equipped with power brakes there is a mushy soft feel before engine is started, and a bit of dead zone pedal travel when car is running as friction material contacts drum or rotor. Manual brakes give a much more positive direct feel without the vacuum assist acting as a middle man when compared to power brakes.

I still think you have an obstruction in the system, and need to blow out all the steel lines, and replace your soft lines.