The answer is always a can of worms or Pandora's box.
What is the purpose of swapping rotors out back? Just for looks? Or performance? I'm guessing the former for most folks?
The generic rear disc kits have this sketchy design where the brake reaction force is applied at the end of these pins. The pins themselves are not really stiff enough to take any large load and therefore won't provide any significant stopping power (too flexible in bending)...by then, the front brakes probably always did all the work. So if it's just for looks, those kits are marginal at best.
The way the OEM brakes work on rotors is, the braking reaction force is taken by the caliper bracket, which is a heavy, stiff casting, actually engineered for the use and abuse.
So take that for what it's worth.
I've never done this, but maybe a NBS truck axle that actually came with rotors is a better option, from 1999-2004. I'm sure there's some fabrication involved so a welder and torch will be required.
Or just forget about rear rotors and do a hydro boost swap, which is a better system anyways over vacuum assist brakes, drum or rotor.
The larger brakes from the 14 bolt 6 lug GMT400 is another upgrade and a direct swap. Still drum, but braking performance improvement nonetheless.
Edit: I'm pretty sure on most of those aftermarket kits you lose parking brakes! Or you have to use some parking brake from like a 70s Eldorado or something goofy like that. So that's another thing to consider.