DON´T use a "Heat-Pal" or similar device running on gasoline or kerosene, it can kill you from carbon monoxide poisoning. Use a thermostatted small electric heater when you are parked at home, connect when you get up and go fetch morning paper, car will be warm after breakfast, thermostat will prevent overheating.
Had an MGB with leather seats. Very cold in 1970´s winters. Bought a strap-on type electric seat heater which plugged into cig lighter. Very useful, especially if you started engine and had it plugged in while clearing frost from windshield, when parked away from home. Combine with a 2-piece skisuit 1 size bigger than usual and "teddy"-insulated low leather boots with zippers and good ski gloves and a knitted cap and you can take any weather. The extra clothing will be easy to put on and take off. When it is not very cold you will do fine with just the boots, gloves and ski-pants (type that you can pull on outside of your regular pants).

Gustafkalle