A lot to consider, for sure. Not having a GP body in hand, I just squeezed the GTO chassis into the Monte for a quick reference. The rear overhang is indeed the first thing I noticed, but it didn't look like a difficult chop. The GP really is kinda short in the rear, it would appear. I hope you can get on this one soon, it will be an interesting project of a subject seldom seen. Excellent detail photos at classiccars.com, including chassis and engine bay. Beautiful cars!