Thanks to your question I read all the responses because I have the same problem. My 1967 Barracuda has 10" rear drums, but none of the parts houses even show that this is possible. They all show only 9" brakes. Anyway, OZ67CUDA said that RockAuto has them and he was right. I could not find them anywhere else. I just received the drum about 3 days ago and it looks fine and measures exactly 10" inside O.D.
I'm still holding my breath until it is installed and working.