if you snag the rear end, you might as well get the front spindles as well so that the bolt pattern will be the same front and back.
the civvie caprices have a 5x4.75 bolt pattern, the 9c1's have 5x5.
the rear end should go right in and will most likely have a more performance gear in it.
i dont know what tranny you have, but since it hasnt been abused, id stick with it.
the engine from the 9c1 will be a 350, but most likely a lot of work to swap in because of the fuel injection.
if you buy the car however, it wont be as bad because all the parts you need will be right there.
since its the same body style, id see if you can get it for cheap and make yourself a frankenstein car.
then sell the 9c1 body with your engine for half of what you bought it for.
my 2 cents