Looking for master cylinder suggestions on somewhat unique setup

Re the Viper MC, it would work for manual brakes if the early-A brake rod bottoms out in the MC piston in the right depth to put the brake pedal and the ideal position, and the bore of the piston fits the rubber bushing on the end of the brake rod. All was true for the Breeze MC I used in my 64 Valiant, with the 2-4 adapter plate (see photos in my post ~10 years ago). If the piston bore varies, you might find an O-ring to fit. No worries with the brake rod popping out of the MC with that protruding cylinder (its purpose?), so the rubber is more to avoid rattles.

The purpose of the large O-ring is to seal vacuum against the booster body. That is for thin pancake boosters which don't have an internal accordion vacuum seal on their output shaft, like classic U.S. ones do. My 1985 M-B booster (Girling) is like that, as are my Chrysler minivans (1995 & 2002). Downside is that if the MC piston leaks, the brake fluid gets into the booster, instead of draining out a hole like with old Mopar boosters. You could just remove that O-ring for drainage (even cut a drain slot). The 1990's Intrepid booster I used on my 1965 Dart is like old Mopar ones, having an output accordion seal.