i'd say if you're drivin an AOD or C4 it'd probably be a nice converter. But gears are right there along side the converter, although most automatics came with at least a 3.27 rear, which isnt quite that bad, at least compared to a 3.08 or 2.72 or whatever that lower gear is.
But above all, i'd have to say the BEST bolt on for under 500 bucks is this baby right here
this is probably cheating - but assuming you already have gears/exhaust - how bout self port lower intake ($50 for cutters), mass air meter, cam, and valve springs? Could probably sneak that in for $500 with gaskets. With a good cam and a lower intake that flows as good as the upper stock, should be knocking on 280 hp . . .