POWER BRAKE VACUUM
Save yourself the problems and diagnose why you think you need power brakes.
MC piston dia can change the feel.
Diferent pads can change the feel.
I have a 67 dart convertible, in the 3100 lb range.
Front KH disk and 10x1.75 rear drum manual brakes, stock MC piston size, Stops just fine.
They can be locked up in a panick stop situation if need be.