Originally Posted by enjenjo
Several vendors offer Ford drums, with no bolt pattern drilled in them. you can have them drilled to fit your pattern.
That said, it is unlikely that the drums can move if the wheels are tightened properly, I would be looking for a bent rim, or a defective tire.
I agree with Enjenjo that the drums aren't moving if properly tightened. If you think they are, I would take a paint marker and mark them and check when you hear the sound. Also does it make a noise if you are moving in reverse? This could also be checked with the paint to see if anything is shifting after braking in that direction. I would be more apt to believe that it is something in the rear housing when it has a load on it than in the brake area.