I use a kerosene heater, and added a CO2 detecter in the shop. It works very well. I can bring a 1600 sq ft shop from 60 degrees to 75 degrees in less than an hour!
Seriously Andy...I think you could bring your shop up to 70 from 50 with two matches and a stick! Come to Iowa in the winter we sun bathe at 50.
When I built my garage one of my nieghbors worked at an AC/HV place. He brought me a furnace out of a house that was being replaced. It was out of a two story house and was 7 years old, I paid him a $100 for it and he installed for another 100.00. He said that small shops actually sell used furnaces a lot....I know I got a deal on the installation but I know that was their going price for the furnace itself. I keep my garage at 40 all the time to prevent rust on my equipment and keep the paints and such from freezing. I "sell" this to the little woman by telling her how nice it is for her to get into a semi warm car....(Clearly this would not work for Andy at 40) I kick it up to 68 when I want to work and it takes about 15 minutes. My garage is 1200 sq, it is well insulated but not overly, and our gas bill has increased about 25.00 per month in Jan and Feb.....I thank my friend for his suggestion and work every chance I get....