If I've got this right, some of the folks building GT40s are insulating the inside of the rear clip....is that going to trap too much heat inside the engine area? Especially at low speeds, I have the impression that this area doesn't ventilate well. I agree though that they run hot inside (small space, proximity of engine, cooling pipes in tunnel, marginal ventilation) and that insulating the passenger space is desirable if you want it to be liveable.
Soundown Inc makes insulation which blocks both heat and noise, might be helpful as well.
Interior spaces which also function as passenger broilers are a characteristic of mid-engine/front cooled cars. A few years ago I had a ride in a Ferrari Boxer during which, if you put your hand out and over the roof, you could feel the huge wash of hot air from the front-mounted cooling system, like a desert wind over the car. Directly after, I had a comparison ride in a 348, which uses rear-mounted radiators. The difference was dramatic. No hot air from the front of the car. One of the reasons that the AC in rear-cooled cars works so much better is that most of the heat stays in the back half of the vehicle. The downside of rear-cooling is that you have less space and elss airflow, so you end up with multiple little fat radiators, and lots of electric fans to push air through them, wiht all the attendant problems. From a comfort standpoint, though, it works better.