May have this figured out.
This is a clutch/brake pedal assembly from a 2011 Mustang.......I did some measuring and it looks like I can easily use the master cylinder and mount it to the '73 pedal assembly. Having the master under the dash solves the issue of clearance around the brake booster.
The bore and stroke of the ford master is nearly identical to that of the 4th gen f-body master, so the ford master should work fine with the gm throwout bearing.