What size is my rear brake drum?

Well I'll tell you this.
Look at the parking brake "levers" the that brake shoe attaches onto.
First see if the shoe is held on with a "pin" and a bendable horseshoe clip, or the lever just goes into the shoe, by a "slot" in the shoe.
That will be the difference in what shoes that you buy.
The pin type was used throughout the 60's, 70's, and the slot type the 80's, 90's. 00's.
Ya look at shoes for B and E body's if you got pin type shoes, or slot, look at vans, trucks.
Look up 2 1/2 X 11 inch shoes.
If your doing wheel cylinders too, they most likely will be 15/16 diameter, as that's the most common, used, wheel cylinder size for the 2 1/2 inch brakes.

Parking Brake Levers 009 (Small).JPG