The core supports are in the front of the car (the front section where the radiator is). The supports that come with kit are round and are thinner than those that are used on the body. The core supports that came with your GTO are most likely square.
The problem with the core supports is that the stand off on the mount is 1 1/8" in diameter and the core support hole is 7/8". I turned the 1 1/8 down to 7/8 and adapted it that way. Not a big job, but then again it is round versus square.
http://www.geocities.com/jims64gto