a 15/16" bore master cyl may work. A lot depends on your pedal ratio. What is on there now? I know it's probably a single piston MC, but if it's 15/16" which is common, try that and see how it works. There is a residual pressure valve inside the single piston MC's, just use it for a short test no freeway driving or the brakes may heat up. The residual pressure valve can be removed, it's inside the MC all the way forward against the outlet. I'd later sub in a dual circuit master cyl for separate F/R systems and better safety.
Get an adjustable rear prop valve and install it in the rear line.

The metric calipers are smaller bore, like 2.5" the 70's cars use 2 15/16" bore which would usually use a 1" bore master cyl.

A 77 chev Malibu with manual brakes uses a 15/16" bore.
any 70's corvette with manual brakes uses a 1" bore.