Brake pedal is really stiff

I assume you verified that the shoes are on correct - short lining on fwd side, long lining on rear side, though wouldn't explain your problem. I agree it doesn't make sense, but "push hard" is qualitative. My 69 Dart had 9" drums w/ manual MC and I don't recall a problem pushing hard enough to skid, plus my wife drove the car. An MC with a smaller piston would make it easier and give the pedal travel you expect.

As a thought, the 10" drum brakes in my 65 Dart are easy. I installed a junkyard booster and MC from a 99 Breeze on 73 Dart brackets w/ proportioning valve for the rear line (plan front discs).

I know more work, but maybe disconnect at each wheel cylinder and blow out the lines w/ air, to insure no rust clogs. I use silicone brake fluid since I have seen massive rust gunk clogs in cars with regular fluid. If you do that, blow alcohol thru to clean the lines then air to dry them. Some diss silicone for often strange reasons, but the U.S. military uses it.