the rear ends out of 9c1 have posi and disc brakes I am told, how much would one of these rear ends go for, what other diffrences are there, I have a 79 caprice, what can I do to make it a better over all car, also are there ant restoration places for the 77 to 90 caprices?

Not all of them have those. You need to get to a 94-96 9c1 to get the disc brakes. Even then you more than likely will not find a limited slip. Prices are dependent upon location and condition. $100-$250 would be my guess. Either way I'd want to rebuild it ($700-$1000) before using it.
The 9c1 rears are somewhat narrower than a ImpSS but I don't recall off hand how that would compare to your cars track width.