What PDB booster and Master for 71 Demon 340?

Any part store: ask for a booster and MC for a 74 Duster with disc brakes, you'll get the right one. If you want to swap to the later (plastic-cap, aluminum body) master cylinder, ask for booster and MC for a 1980 Aspen. As long as you have the right booster or use the MP 2 to 4 bolt adapter, you can use ANY 1967+ disc master cylinder to at least the 1990s.