Few things:

It takes 465 crank hp to beat a GT500, this is something I've tested back to back.

4.6l 3v engines are safe to 500 crank hp.

The transmission on the manual will blow at 465 hp, but the transmission on the auto will be fine. With the manual transmission you need about $1000 worth of parts to make it safe.

A GT500 will always be about 12% slower than a GT at everything (handing acceleration etc).

A GT500 actually makes 520 at the crank not 500.

 

your all missing... stock to stock.. all waranty in the gt500 kills the rest
If i had a gt500 i would maybe do pully tune intake and leave it at that other then suspension and maybe a few other things. but otherwise leave the motor alone other then quick pull off stuff incase crap goes wrong....
i just had crap go wrong and i was thinking it would probably take 4 or so hours to return to stock to bring it to the stealership to fix... THANK GOD FOR SECA