Burbites,
I found on mine that Insulation and weatherstrip is the key.
I made wooden plywood bows about 2" thick and followed the contour of the roof and sandwiched Foil/Jute insulation foil
side up.Then put in the headliner.Same on the firewall and floorboard.Used the cheap bubble stuff in the doors glued to the outer skin.
My heater is the fresh-air version without using the mixer valve.Put in a 12 volt motor from NAPA and a reo-stat switch.
in 20d weather the entire cab is very comfortable.
