Master Cylinder Choice.

A 76 Dart MC should bolt right up, and I think the pedal will be in the same place. The attachment to the pedal should be the same. I say this because I installed power brake brackets from a ~73 Dart in my 65 Dart, but used a booster/MC from a 96 Breeze to save weight. I got an adjustable proportioning valve off ebay for the rear, ~$25 and looks just like the ~$50 from Summit, etc. I plan front disk brakes later, which explains the booster. The pedal is almost too easy with the current 10" drum brakes.

For manual brakes, you want a MC with a smaller bore. I would guess <1"D. If you can find a factory one for 73+ manual disk brakes that should work. Personally, I wouldn't fool with an old cast-iron MC, but would use a later aluminum MC. They look pretty generic for most cars. However, you need an adapter plate to the 2 bolts (~$50) or maybe one could just drill 2 holes in the firewall (anyone tried?). I wouldn't use a factory proportioning since they are fixed and designed for a specific car. An adjustable one is cheaper anyway.