which master cylinder?

The problem is that the brake part manufacturer application listings for A-body disc brakes are thoroughly screwed up. In theory, there should be two 1967-72 disc brake master cylinders: one for Manual with a 1-1/32" bore, and one for Power with a 15/16" bore (otherwise interchangeable). But you will find a lot more variations in part numbers than just those two, and some of the listed items will actually be drum brake masters. Annoyingly, most store sites don't give the bore dimension, but you can Google the part number and find a manufacturer listing that will show it (Cardone or Bendix, for example). That said, it is frequently recommended that you use the Power master cylinder for Manual disc brakes — the smaller bore reduces the pedal effort while slightly increasing the pedal travel — most people today seem to find that a reasonable trade off. I have one car with each, and I can tell you that the 15/16 bore MC feels plenty hard — the 1-1/32 bore feels like you need both feet (great for threshold braking at the track, but startling if you step into it from a modern vehicle). There are also other versions of the same basic master cylinder with other bore sizes — 1" is common on B-bodies, and 1-1/8 is also encountered. So I don't trust the application listings — I research the specs for the part shown to ensure it has the bore I want.