GM used ‘corporate’ engines in all brands. Some Pontiacs had Buick engines, some Buicks had Oldsmobile engines, etc, etc. I wouldn’t shit you, you got a Chevy motor.
My thread on here is http://www.pittspeed.com/forums/showthread.php?t=31040
My website has more pics on it: www.akvalley.net/~ss & click on the GTO.