need a pic of master cylinder with manual setup

OP, did you car come originally with the disc brakes up front? Or, have these been changed out to the later '73-'76 disc brakes? I ask because the earlier disc brakes were Kelsey-Hayes and the later were not, and they are different; MC's are somewhat universal buuut...... If you don't know which you have, can share a pix of one of the front calipers or tell us if it has one pisotn on one side only, or 4 pistons, 2 in each side of the caliper?

If Autozone is showing a single MC, that is surely wrong; the single MC's went away after '66.

If it turns out that you have the later '73-'76 disc brakes, then the disc/drum MC for power brakes is 15/16" and the same for manual brakes is 1-1/32". We used the power brakes version (15/16") with our '73-'76 disc brakes and 11"x2-1/2" rear drums and it worked fine. (NAPA Part# NMC M2122) It looks like the one that 66fyssh shows in post #2. With improved front pads to set most of the F-to-R proportioning, it is even better.

BTW, do you know what size rear drum brakes that you have? (This has to do with the proportioning front-to-rear.)