I have a basic 73-76 A-body disk brake setup on my car, but with the later model 2.75" piston front calipers and 11.75" rotors.

I have the 15/16" master cylinder from Dr Diff. Stops well and doesn't even come close to bottoming the master cylinder. A-bodies with power brakes came with this size from the factory, and for some reason manual brakes came with 1 1/32" Master cylinders.

I would run the 15/16".