I have had my a$$ chewed royally by total strangers for modifying my Burb. They mostly tell me that I have ruined a classic truck. I built it to go to local cruise ins and if I have to keep up with Interstate traffic I can. It has 283/TH350, power front discs, seat belts, power steering, and IFS none of which is stock or original. I consider it to be as safe as my 06 Silverado. In Wisconsin I have the Hobbyist plates and one restriction is I can't drive it in the month of January so the under seat heater is pretty low on my list. If you want to put in a rear area heater I hope you find the stuff you are looking for. It sounds like a good idea for NH.
Weeds