If I were you, I would do a C4 corvette rearend. It is MOSTLY a complete unit, the only "floating" parts are the control arms, but they have a nice mounting bracket that bolts on that will line up real nice with the rear frame rails.
http://cgi.ebay.com/91-96-Corvette-C...item1e5feef914
My next-door neighbor at the shop dismantles C4's and has about 6 of these right now. Dana 36 came in automatics and Dana 44 in stick cars. The 36 would do fine for a street Firebird. Guys put these in all kinds of cars.