Hi jisturm!
For a Turbo (itself) rebuilt, I use Turbo City, in the city of Orange, Calif. I don't have their address or phone number right now, but if you need it, let me know. I can get it! It has been quite a while since I had one rebuilt, so I don't know a price.
As for the rest of the engine. If it needs it, any shop should be able to do it.
I don't know what city you live in/near, but most major cities have a turbo rebuilding place. Check your phone book. You may have to check out marine engine rebuilding shops as well, although they will be more. Here in Los Angeles, there are quite a few shops popping up (we can thank the ricers for that-one good thing about them-Ha!)
It will never outperfom a 400/455 pontiac, but these cars can hold their own in the twisties/turnies!! Gotta love the WS6!
Nunzi out in New York does some small performance mods, you may want to contact him. I'm not sure it would be worth it.
Like I said before, check out
http://www.transamcountry.com
They have links to check out and such!
Good luck,
George :twisted: