Thinking of converting to Manual Brakes

Try your current MC. You probably don't want a bigger piston bore with disc brakes, since that would make the pedal harder to push. Kind of confused by what you say, because I recall that power brake MC's had a SMALLER bore. That sounds counter-intuitive until you consider the lever arrangement in the booster. I expect that most/all factory disc cars came with power brakes, so you are going off-book. If you change the MC, consider a modern aluminum MC w/ 2 bolts to save weight. A 1" D or smaller bore sounds right.