From what I've seen the most immediate differences from the original GTO is the length of the bonnet and the rear hatch. The GTO is longer in the hood or appears to be. It may also be the GTO is a bit shorter in the doors. The GTO doesn't have a rear hatch, just a small rectangular boot lid but I did see someone did a great conversion on the rear Datsun hatch area to make it look the same.
Gotta love the shape of that back end.
Cheers