Ironworks chassis!!!!
They just came out with a chassis that should fit and it looks damn sweet. Look it up on the forums (for 64-72 Chevelle)
LS3 + T56 should be relatively easy to come buy. Price depends on what you want. New or used? Stock or modded?
For example!
You could even get a LS6/T56 drop out of a CTS-V or something.
https://lateral-g.net/forums/showthread.php4?t=30134
Good luck and let's see some pics!!!!