I have a regular a propane house furnace that vents out the back wall, with a 500 gallon tank behind the garage and a thermostat that I can set to any temp I want. Works great.
My cars are locked up in a well insulated garage all Winter, with southern facing windows in 3 doors and a black roof. It has a cement floor. No heat at all. Odd, but there is a half gallon jug of water next to the Model A which never freezes. It's on the floor too! Don't get me wrong though, It's too doggone cold for me to work on the cars.