Seems to me starting with the change to single piston disc brakes in 73 Mopar added a booster so manual brakes either point to added discs or removed booster... Either way the M/C bore size becomes a question...
I had a factory stock 1973 Duster that had very few options...318, 904, manual front disc and an AM radio. That was it. Rubber floor mats, heater/defroster, no vinyl top or light group.
Manual front disc A body cars may not be the majority but they did exist. I've converted several drum brake cars to front disc using no booster and they usually have excellent braking. I had a 73 Challenger that didn't stop well with manual disc. Hard pedal and poor stopping.
My '70 Charger has had a few different brake configurations including a manual 4 wheel disc setup. That also left me with hard pedal and poor braking.....this was the same with 4 different master cylinders of different sizes.