Hi Glen, good luck with your quest
I have a heater in the 289 JCF car, installed in the passenger foot well. Last year Brent & I went on a 100mile tour in late Sep. starting to get cold here, by that time of the year (30-40 degree F). By the end of the day, Brent's right foot was warm
We had the top on and side curtain's closed, it was somewhat warm, at least you couldn't see your breath. I would say the engine heat helped too. Just part of the fun of owning our Snakes
Cheers TommyRot.