What specifically are the brakes doing?
-Are you able to lock up any/all of the wheels during braking?
-You say the pedal is hard, what is the travel like? Can you depress the pedal at least a few inches with effort? Or is it rock hard and not moving?
-have you followed the bedding procedure for the new brake pads/shoes?
Do you have any pictures of how everything is hooked up?
I’m not trying to beat you up, but it seems like you’re making very general recommendations with a lot of speculation on master cylinder bore etc. There’s a lot of advice being given for the OP right now -which is great, but if we overwhelm the OP with completely unrelated information or speculation it stops being helpful.
A quick search here on FABO will show you that the factory master cylinders ranged from 15/16 to 1-1/32”, and that a lot of the members running manual disks prefer the 15/16” master cylinder bore diameter. But any master cylinder in that range should stop the car just fine, like you said, it’s just fluid dynamics and the components the OP is running have been shown to work great on other cars.
And it’s entirely too soon to say it’s the back brakes, because we simply don’t have enough evidence to say that. The OP has a bunch of new parts, a bunch of old parts that sat for awhile, and there are lots of different issues that could be causing the issue. 70-80% of the braking is done by the front, so if the braking is scary it’s much more likely a problem with the front, or a systemic issue affecting everything
Sounds like it’s a new one from DoctorDiff, which would mean it’s either 15/16” or 1-1/32”, as those are his options. Either should work fine as long as everything is hooked up correctly.