DRUM SIZE

If it's factory, you could have 9" or 10" brakes. The front would be either 9 X 2 1/2", or 10 X 2 1/4". The rears would be 9" X 2" or 10" X 1 3/4". Usually the 6 cyl cars came with 9" brakes and the V8 cars came with the 10". If your car came with disc brakes in the front, they would have had 10" brakes in the rear. As mentioned, the diameter is measured on the inside of the drum surface....where the shoes ride. Measuring the width of the drum can be tricky as there were several different manufacturers of drums and they didn't all have the same width shoe surface. Assuming the exiting shoes are correct, just measure the width of the shoe to get a replacement.