Brake backing plate interchange???

[MP33]

I also posted this on the 67-72 board:

I'm installing a 71-72 rearend in my 63 C-10



Need to know if the 79 C-10 backing plates that I have will work to replace the rusted/shot 71-72 backing plates. I just have the backing plates on the 79 rearend, no drums,shoes or anything else. How do I determine if they are 2" like my 71-72 or the heavy duty brakes? I doesn't matter to me if they are the heavy duty or 2" but I need to know what to buy tomorrow.

Update: I measured from the axle face(Where the drum mounts) to the back of the backing plate and the 71-72 is 3.375" and the 79 is 3.75"

I do have a 64-70 12 bolt with complete brakes and that one measures within a 1/16" of the 71-72. could I use those backing plates and brake guts from the 64-70 rearend and just get new 71-72 5 lug drums???