For some time now, I've been having an ongoing problem with too much heat in the passenger compartment in my '70 Coupe. Every time I took it out for a ride, once the engine got hot, heat would stream into the cabin, cook my gas pedal and foot, hot air pouring under the steering wheel, and the radio and it's metal surround getting so hot I couldn't touch them. I tried everything from plugging all of the holes in the fire wall to blocking the inlet from the heater box to disconnecting the tubes running through the dash to installing layers of Koolmat. Nothing stopped the heat from coming in.
Frustrated, I was talking to a friend who has a '70 OTS. His car is fine, no heat problem at all. So we started talking about the differences. We realized the main difference is that my car has factory air conditioning, and his does not. So he grabbed a light and mirror and followed the AC lines back toward the cabin. They run into the transmission tunnel. Using the mirror and light, he spotted a huge hole where the AC lines go into the cabin, right above the transmission. This hole allowed super-heated air coming off the engine and headers to go directly into the dash area right behind the radio and angled toward the driver's side. Heat problem explained.
We sealed up the hole around the AC lines with some foam and sealant, from both sides. No more heat problem!!!! I can now drive the car without getting cooked! If anyone else is having a heat problem, and you've got AC in your car, this might well be your problem.