Rear disc conversion

I have a 1989 caprice and I just purchased a rear disc brake conversion for it. My issue is I wanna upgrade my calipers and rotors but I dont know what's compatible. The kit came with rotors from a gmc safari and calipers from a 79 eldorado.

That's a messed up kit you have.  It's pieced together from different years parts and will be a PITA to replace the high wear parts.  Doing an aftermarket brake conversion is a lot more difficult than most people are aware of.  Depending on the kit you purchased, you may even have to pull both axles to mount the caliper brackets.  Guaranteed you're going to spend hours and hours just trying to shim the parts into proper alignment.  Also, did it come with a new combo valve?  Probably not.  You'll need one for rear disc brakes. What about the parking brake? Does the kit have provisions for that?

Seriously, you're better off sending the parts back and simply swap out the whole rear end to a Posi disc brake rear with the proper combo valve.
If you're insistent on only doing the brakes, use the 2nd Gen Blazer setup.   Super cheap on cost and they're a direct bolt on, no shims required.  

Thanks for the response! Actually the kit is allready installed and it worked well my issue now is I wanna upgrade the calipers to wilwood but I dont know which ones are compatible with the bracket and calipers that came with my kit which are from a 1979 eldorado. I wanted to upgrade my rotors also but I dont know which ones to get or what rotors are the same as the ones I have which are from a gmc safari.

It sounds like you need to consult with a Wilwood rep.