Seems how your shop is already built the best thing you could do is install an infrared (radiant) heater overhead. They heat the objects in the shop which in turn heat the rest of the shop. I have had convection, diesel, home furnaces and radiant over the years and radiant is the best. It will cost more than a used house furnace but it is well worth it. With a house furnace you tend to get alot of heat wherever the vents are and that is about it. I know lots of people with them in Canada and have never heard a bad thing about them.
Beyond heating it is also important to have good insulation and a good door seal.
Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid-in-sideways, totally worn out, shouting.."holy crap.... what a ride!"