Rear drum should be around eleven inches inside diameter with a two inch wide brake shoe. The shoes have a different length as that is how you distinguish the front shoe from the back shoe. The shoe with the longer material on it goes to the back of the drum, short one up front.
Other wise I have no idea what you are talking about.
If one drum is about two inches in diameter smaller than the other that would be hard to explain how it goy that way; but the Chevelle, Nova, and Camaro, as well as later Monte Carlos, etc. all had nine and a half inch drums on the back. Heavier GVW cars and the light pick-up trucks like the Impala (all B-body cars), and D-body Cadillac limos got an eleven inch drum. Heavy Duty three quarter ton and one ton pickups got a thirteen inch drum with two and a half inch wide shoes.
All GM's with Spicer rear ends (the 10 bolt and the 12 bolt) used interchangeable backing plates. The backing plate determines the brake size and width of the shoes. So one from a smaller car could have been bolted on by mistake.