A few years ago I was swapping my 1970 2wd C/10 from manual 4 wheel drums to a factory style 1971 front disc rear drum, 5 lug, power brake setup. I did some research on the forum and eventually purchased a restored Delco/Moraine booster and a rebuilt master cylinder (for a 1971 2wd C/10) from Autozone part number M1494 that looks like the Delco unit posted earlier by 1970cstblazer. The one I got from Autozone is no longer listed on their site. Your thread has got me thinking about my setup and I was looking around the forum and ran across someone saying that GM used two different suppliers for these trucks, either Delco or Bendix. They said if you had a Delco booster you would have a single side flip up bail master cylinder and if you had a Bendix booster you would have a double end-to-end flip up master cylinder. I don't know if that's correct since my truck didn't originally have either. However, NAPA lists a master cylinder that looks like my double bail unit
here. Dorman also offers one that looks correct
here but we know pictures can be outdated. I don't know if either of these are what you're looking for but it gives you a couple things to look at, if you haven't already. I also want to use an original/correct cast iron prop valve and have bought a couple different units. Once I figure out which one is correct I plan to have it rebuilt.