You get to make a trade-off between pedal effort and pedal travel. A smaller master cylinder bore will reduce pedal effort at the expense of increased travel. Are you using large or small piston calipers? What size rear brakes and what size rear wheel cylinders?
My brake donor car (75 Swinger) had manual disk brakes. The master cylinder bore was 1 1/32" and although I only drove the car a couple miles, I recall the pedal to be hard, but manageable.
That said, on my 65 the brakes will go back together with the larger bore calipers (2.75" vs 2.6", with larger 11.75" rotors) and a super common 15/16" bore master cylinder. If rear lockup is an issue (doubtful) I will reduce the rear wheel cylinder size (10" drums with 13/16").