i can tell you that not all 3.8s were created equal. the motor you have in your GTP is not much at all like the 3.8s they put in the GNs back in the '80s. however, that's not to say that you can't find a company that makes a 'kit' to fit your car. since it's not very popular, comparatively with say a honduh civic, there probably isn't a company that makes a true bolt on kit...
after a little bit of research:
http://www.3800performance.com/ct_turbo_kit.htm
you might also consider a supercharger... less underhood heat and quieter operation. since you probably won't be pushing more than 10 lbs anyway (this is a street car, right?
) a supercharger may end up cheaper and are tons easier to install (if you can't find a turbo 'kit') - no need for fancy downpipes and exhaust rerouting.
at that level, you'd get the same type of performance out of a supercharger.