So I dug out the 72 Plymouth FSM (don't have my 72 Dodge one handy). Shows 1 1/32" MC bore for Valiant with discs. However, it only came with 10" rear drums and they are different than the 9" drums in more than just diameter.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.