Yeah your right on the 7.5 rear end. GT's havethe 8.8i would keep the v6 engine and put forged internals. and a fat turbo with cam's and head's. it would be fast a balls. also the v6's i think have the 7.5 rear you would need to change it out to the 8.8 which is in the Gt's. i would also redo your suspencion to. shocks struts, springs the works. and a full exhaust system. and also some meety tires.
My suggest like stated above would be to get a GT and build that. If your get a motor built I would want it to atlest hold 600hp no problem. Also for look into a 4r70w or 4r75w....The c4 runs hot and I would only suggest for a track/MILD street car.