Newer aluminum MC's have 2 bolts. I plan to put one on my 64 Valiant to get a dual system, and I don't like heavy, rust-prone cast iron. I was going to just drill 2 holes thru the firewall, but will probably get another "2 to 4 bolt" adapter plate. I paid ~$30 for the last one (ebay), but you can pay $80 if you prefer buying from speed catalogs. I used the first one to mount a new MC to a new factory booster on my C-body.
Re MC, many here use an ~85 Dodge truck one, but I expect many would fit. I used one for a 95-99 Breeze w/ ABS on all 3 of my Mopars. On my 65 Dart, I didn't need the adapter plate since I used an entire booster/MC from a Breeze on Dart stand-off brackets. Why ABS? Because those have only 2 ports. Why 99-? Because most later MC's have bubble flare ports, though you can actually form those (see youtube). All my cars have drums, though I plan front disks for the Dart, hence the booster.
Re size, mine are 7/8"D bore, and work fine. I recall that factory manual brakes are 1"D bore, but check rockauto. I wouldn't go bigger, unless you have a really strong leg. rock shows a smaller bore for power brakes in most A-body years. That sounds reversed, but not after you understand the lever ratio in the A-body bracket. A smaller bore makes an easier pedal, but more pedal travel. I use DOT 5 because I don't like rust inside my brake system either.