There are many things that will cause a hard pedal and poor stopping. The mismatched master cylinder is most likely a major factor. Disc/drum systems are supposed to have a dual reservoir master cylinder with the larger reservoir feeding the front discs as they use more volume than drums. In any case, single masters are outdated and potentially dangerous (if you have a leak anywhere in the system you lose ALL your brakes) and should always be replaced by a dual. It's certainly not matched with your setup.
Without knowing your exact setup, it's hard to tell which one to get. On mine and many others, the replacement for a mid 70's Mustang and similar models is used (GM front disc/S-10 rear drum). For front disc conversions, the Corvette master is popular also.
I would go through the entire system, because anyone that would use a single master with your setup is not very savvy with the way braking systems should work.