72 3/4 ton 4x4 rear drums HELP

[StingRay]

Hi guys I need help trying to figure out the rear drums for a 72 3/4 4X4 for a guy I know. Here's what we know:

72 truck with a 12" x 2" brake shoe face and the drums are about 5" deep in total. Upon trying to find new replacements he keeps coming up with a drum for 12" x 2 1/2" or slightly larger shoe and they don't fit. I don't know specifically what diff he has or that is exactly the one it came with the truck for sure but it is probably a genuine 67-72 rear diff.

If anybody knows what we need to look for in the parts books to find the correct one it would be greatly appreciated.