Did this on my 71 FB resto....will do the same when I get there with the 69.

I made my own "Lixzardskin", which is basically an epoxy paint mixed with a high volume of microballoons (glass microsheres used for filling/bulking fibreglass when fairing). The microballoons act as an insulator....2 coats of this, then I covered the floor, toe boards and up onto the firewall with Dynamat. The firewall inside was covered with Dynapad, but most of the heat comes through the floor when you have headers, as well all know. Over the Dynamat on the floor, I used Dynaliner on the floor pan footwells, then a layer of underfelt, then carpet. I also insulated the ceiling of the interior with a layer of alluminium foil backed bubble mat....ver ylight ,and an excellent insulator.
When driving, I had no sensation of heat through the floor or firewall, and when you stepped into the car when it was parked in the sun, it was pleasant inside.