I was gonna suggest the cabin heat was from your exhaust but was beat to it. You could put some ceramic coating on the exhaust until it is behind the driver, that will move the heat further back.
Pull the plugs and check them. Do you have a problem of restarting the car right away after it is warm? I had something similar in mine and it turned out to be percolation in the fuel bowls. I put on a 1/2 spacer and used some of that heat shielding material that people use for starters and zip tied it to the fuel line that ran over the manifold to the carb. Cured all problems. Also, have you tuned the carb? Changed jets? I also had a time when I was running rich and too much gas was being dumped into the engine cause similar symptoms. Yeah, the heat thing is probably the headers passing heat to the tranny. My headers would through enough heat through the floorboards that I never missed the heater iin the winter when the core leaked (bypassed it). make sure all of the spark plug wires are routed away from the headers and that none of them are brittle or have cracks or holes from the heat (yeah, dealt with that one too, similar symptoms). Good luck.