If you do want to investigate the cast route: contact 5.0LMiata (Brett) from the other board (not sure if he's on here or not). He works for WesCast, they make engine manifolds for GM. If anybody could hook you up, it would be him.

I worked for Western Foundry (now WesCast) in Brantford during a couple of work terms. Really cool place!

http://www.wescast.com/global-locations/