Do older rear brakes fit?

As Rusty stated above the backing plates are different on the taper axle. The drums are reusable after cutting the swedge in lug studs. Any 1965-up to mid 70's 10 1/2'' backing plates are usable. Not sure of the year that the wheel cylinders changed ( the mounting point hole was more round) to a squareish design. Maybe when the later 8 1/4 axle was more common than the 8 3/4 went away. Shoes changed the mounting of the parking brake lever to from a round hole clip design to the hooked lever (maybe late 70's ?) Hardware kits may or may not have all the same parts and maybe the reason the change in part number.
As you stated the Axle flange to backing plate from a body 10x 1 3/4'' axle to the 10x 2 1/2'' distance is different.
At one time A body conversion axles small bolt to big bolt pattern 4 1/2'' were designed for the 10x 1 3/4 shoes .
I believe all made are now are for the 2 1/2'' shoe design.