Does your upper pan have the rear tab shown in the image below? That's what it looks like to me...
The GTO hood is different underneath but my upper does have a couple of similar tabs at the rear that hook on to the under-hood structure to hold it up while drilling the screwholes, etc. It's a big help if you're doing it ON the car. If you take the hood off, you may not even need it. On the other hand, whenever you change that upper seal in the future, you'll want that support back there.
As far as cutting the underhood support, I did mine on the car but I had no one to help me get the hood off. Definitely would have been easier cutting that way if I had some help but it wasn't terribly hard even on the car. Just had to cover the engine compartment up well.
I used a 4-1/2" electric variable speed cutoff wheel. However you do it, take a little off at a time, fitting and marking the hood as you go.
Put the foam as far outboard as you can get it on the flat, upper-pan seal mounting surface. If you allow it too far inboard, it'll show in the scoop openings and look like crap.