If you haven't done so, start with insulation. Regardless of your heat source, you're going to lose a lot of it through that door. You want to try and normalize the temperature a bit.
After that I would look at leveraging a radiant heater. Unless you can ensure good air flow, look at electric instead of gas. If you must get a gas based one, look for one rated for indoor use. Radiant heaters focus the heat much better than other types in that space and you're not wasting energy heating the entire garage. Be vary cautious of damaging your cabs or causing fires though.
If radiant isn't your bag, I would look at forced air heat. I don't like them as much, they're too noisy and they never seem to last more than a year or two until the heating element falls apart. But they are generally safer (relatively speaking) than radiant heaters.
Don't bother with convection heaters, they'll be a waste of energy and your cabs will do an equally, if not better, job heating up your garage.
Chad, Doh! I didn't think of that. Oh well, I don't want to delete all of that.