Sounds fun! I have a buddy who builds rails. He has built a couple of them with the TBird Super Coupe motors on them mated to a transaxle. Not sure what transaxle he used, but I DO know they are a BLAST to drive!!
I would think that any LH 3.2/3.5 would be a nice setup as well. Be sure to get all the electronics as was mentioned above for whatever year you use!