hey poncho mike, welcome to the py forum. I have a 65 with the 15x7 rally repros and I can run BFG radial TA's. Front 215/70-15, Rear 255/60-15. This will keep your tire diameter around 27.1 -27-3 inches which is similar to what GTO's had originally but with the added benefit of wide radials. If you want to have the same size all around you may be able to squeeze 225/70-15 in front and back but this changes the diameter and the front may rub, which is never good. My cars previous owner had 245/60-15 all around and the front fenders got warped from the tires rubbing.
Hope that helps, email me if you want more info,
[email protected].
Mike