Some of the things I can think of that would need to be swapped.
Tailights and tail panel
All the badging.
Not sure if you have to drill for new badges. I would thinks so, because the LeMans badges are a lot bigger.
Relocate and change the front parking/turn lights from the bumper to the grill,
Remove the pontiac badge from the header panel and repair holes,
Remove the 326 emblem from the front fenders and repair holes,
Patch the ribs in the front quarter,
Change the rocker chrome to the wider 67 style, the front fender badge mounts on the rocker chrome with the front and rear extensions.
Repair the lemans holes on the rear quarter,
Add the GTO badge to the rear quarters and deck lid,