I expect you already have a booster for drum brakes since my base 1965 Newport 2 bbl does. The MC just makes line pressure, which is the same for disks and drums, but disks generally require more pressure because drum shoes are self-amplifying, so get a smaller MC bore for less pedal effort. I recall 1"D was factory for drums or disks, so 15/16" or better 7/8" would be good. Best to change to a dual MC for more safety. You can do that w/ a 2-4 bolt adapter plate ($30 ebay). I have a 95-99 Breeze ABS MC on all 3 of my old Mopars (1 C, 2 A's). Dr Diff sells plate w/ MC for ~$100, which many A-body guys use. His MC is basically the mid-1980's Dodge truck one, but offers a smaller bore. You must have a proportioning valve w/ front disks/rear drums. An adjustable one is ~$30 ebay. Adjust so the fronts skid just before the rears in a wet parking lot. I kept my drum distribution block as a tee for the fronts (plugged rear port) and ran a separate tube from MC to rears (coupler to OE rear tube).
Re "why disks?". Some say they can't stop fast enough with drums, but that is strange Physics. The tires stop the car, so if your brakes can bring them to the max braking point (just before tires skid), and equally L & R, that is the best you can do. This presumes the factory picked the rear wheel cylinders so the rear braking is also just before that threshold. Most as-found drums lack proper maintenance, so don't compare old drums w/ new disks. Drums downfall is they take longer to cool, so riding the brakes downhill (stupid), twisty roads, or stops from very high speed may cause fade (friction material melts). Otherwise, no scientific reason why drums couldn't stop a car just a fast. Don't quote braking spec's from 1960 w/ bias-ply tires. Sticky tires are how some modern muscle cars stop from 60 mph in <100 ft (saw one spec 90 ft recently).
