My '68 OTS would also get very hot. When the engine and interior were out, I added a metal foil backed underlayer plus strips of Dynamat to the entire interior of the car. Gaps around the transmission tunnel was allowing lots of hot air to get in to the cabin. I also found one of the OEM exhaust heat shields was missing plus I wrapped the exterior surface of the transmission tunnel in a thin, self adhesive metallic insulation layer (thanks John Carey). I have also kept the transmission fill rubber bung covered with the insulation layer which makes transmission oil filling an exhaust off job but the whole thing has completely transformed the driving experience and comfort level. It’s a fairly involved job but even on a long drive in the CA summer, the inside now remains very comfortable.
Andrew