Another place to consider that I have heard good things about is Muscle Car Metal. I don't have any direct experience with them, but there are a couple people over on Team Camaro Tech that have used these shells from Tony. I wish I had known about them before we ended up replacing so much of the sheet metal on my firebird as it would have been a better starting point, or at least maybe sped up the process. His are refurbished originals, so they come with titles.
http://www.musclecarmetal.com/home2.asp