Actually yes it will. Caprice/Impalas from 1977-1996 Share the same basic chassis. It will fit, also how do I know for a fact? I have a 9c1 rear end in my 88 caprice. Also just about any and all Suspension parts from a 91-96 caprice/impala will work with a 77-90 caprice/impala.
However you will now have two different bolt patterns, unless you have a 80's 9c1 police package caprice which I believe did come with the same 5x5 bolt pattern as the 91's caprice/impalas