Rubbing is exactly the problem. Having said that, every GT40 is different. Especially in the front clearance. This will depend on wheel offset, final fit of bodywork, especially the sills, final trim of wheel well arches and sill lip, ride height, spring rates, and even shock settings. If you have a 1/2" or less of clearance anywhere, lock to lock, then you will need to really check it all out dynamically. 3/4 - 1 inch or more and I think you will be OK.
What I am trying to say is a 1/2 inch taller can be a lot. Not always, but depending on your car, maybe. In any case you don't what tire rub, lock to lock, full bump/droop. None nadda.
See if you can work with a tire shop and try them on for fit without running them. You will only need to try one front tire, no balance, just fit. They shouldn't gripe at that. All you need for the other side is a tire of the same diameter.
I did mine in the end at home. Put the car on a jack at the front and took the springs (front) off the shocks. Then lowered the car until I had the correct ride height, turned the wheel back and forth, jacked it down to full bump and checked it again. Trimmed fiberglass to fit. All good now.