71 Dart, buying brakes, ugh

The diameter should be stamped on the edge of the outside of the drums. Might have to clean off some of the green paint but it should say something like "Max dia 9.060" or similar. I'm guessing you have 9" by 2" brakes. The books probably list only 10" brakes but those look like 9". Is it a 6 cylinder car? You might have to try getting brakes to fit an earlier A body. The computers don't necessarily list everything. If you take a drum to the store with you any decent parts guy ought to be able to get what you need. Try talking to the oldest guy working there. They've usually seen everything & know more than just what the computer tells them.