I believe the preferred stock manifold of choice for mucho power is actually the GEN I "4-rib" manifold with extruded aluminum runners rathen than the later models with cast runners. At least, that was what I was told by a very experienced Viper Tech who was building a money-no-object GTS a couple years ago for a customer.
I think the 4-rib has a bit more plenum volume, and there's a theory that could be worth just a bit more HP. Looking at the runners of both, I don't see a particular reason to to scour the Earth for an early 4-rib. The runners look so large anyway..