The doors are not the same.
Yes
Yes. 91-96 impala, roadmaster, caprice, and fleetwood rear end are all the same and will fit with no modifications.
Not likely as GM restyled the Caprice/Impala slightly in 1980.
1977-85 Impala, 1977-90 Caprice
Any 1977-85 Impala or 1977-90 Caprice will work
Yes
Since they're basically the same platform, it should fit without much problem.
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
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Any other grille from a 1977-79 Caprice/Impala would be a good fit
Any door from a 1977-79 Caprice/Impala would fit (assuming it's a similar bodystyle)
The tail lights are interchangeable, the only difference is the chrome trim that was standard on the Caprice