So, "original" doesn't matter?
I just went through this on a 67 B body.
A Diplomat master cylinder will work OK with a 1.031 bore.
(But you would need the adapter to go aluminum)
But I was unhappy with the A body disk and that master.
I would recommend upgrading to the 11.75 1978 Cordoba disk
(O'Reilly or Summit)
You would only need these if you have everything else.
https://www.doctordiff.com/disc-brake-slider-caliper-mounts-backets-for-11-3-4-rotor-upgrade.html
And this either way over the Diplomat master.
15/16 bore.
I believe that you have 1/2X20 and 9/16X20 lines?
https://www.doctordiff.com/aluminum-master-cylinder-kit.html