You need to check out the
http://www.impalassforum.com/vBulletin/ for the best source of B-body info.
Good luck finding seats as the good ones are very expensive. I'd try to find some C5 or GTO seats. The late 90's Bonneville seats are a near drop in, but those are also getting hard to find in decent shape.
IMO, the single best modification to my Impala is the T56 six speed.
It is so much more fun to drive now.
Information on my Impala SS
Good luck in your search.
Randy