You'll have a hard time finding a 5.7 with a manual trans from the factory. The 5.7 GTAs came from the factorywith the 700R-4 automatic. The 5 speed was not available as an option with the 5.7 TPI engine. The 5.0 on the other hand came with either manual or auto.
Whatever you can get your hands on will be a great car anyways, and fun to drive too. Not very comfortable on rough roads though.
-Ozzy