I have a 91 RS with Drums.
If this is a fairly easy swap I can get a disk rear-end from an 87 IROC.
So far It looks like I will need:

prop-valve
E-brake cables
flexible brake lines with different ends???
new hard lines?

Replacing the hard lines sounds like a pain without a lift.
Also, is there a difference between 91 and 87 pad sizes (when they need to be replaced)?

 

The 87 rear (unless the former owner updated it) will have the crap Delco Moraine disc brakes. I'd try and find a rear with the PBR brakes unless you already planned to do a brake swap as well.

 

He only wants $75 for it and he said its a posi. I wasn't planning on doing a brake upgrade for a year or two. Thought those would ge me by untill then.... or are they that bad?
Still worth it?

 

Your drum brakes will work better than the '87 disc brakes. If you have aluminum drums, I'd recommend staying with those until you upgrade to some good rear brakes (1LE/LT1/LS1 etc.).

 

I had bad problem with 87 rear disc. Evertime apply brakes it would shake and make awful nosie. at the time wanted drums from factory. But converted to ls1 and looks good but noww i need to do the fronts.

 

I'm running the Delco iron calipers on a pair of salvage yard rotors and pads (got them with the rotors). I was planning on using the pads just long enough to clean off the rotors, but since the brakes are so much better than the old iron drums, I'll probably keep them till they wear out. I can actually lock up all four wheels if I need to...could have never done that with the old drums, but that's with the added complications that my stock combination valve was probably bad.

Keep in mind that the 87 model year was running the weaker axleshafts, if memory serves me correctly. I did this exact swap a few weeks ago, but only because I'm still running ~200 HP @ crank. I will HAVE TO upgrade the rear end when I'm pushing anything more.