For a quick reference, take a look at the illustration in the LMC catalog.
The original step-side rear bumper mounting brackets, are essentially, made of flat bar stock, whereas the fleet-side rear bumper brackets are basically C-channel stock.
How do you identify a fleetside set of brackets from a stepside?
I have a 66 fleetside with the bed off. I can send you photos of the bumper, brackets and covers if you want. Send me your email address by PM and I'll send'em right away.