I wouldnt use any of the propane type heaters, they put out too much crap in the air unless you have a good source of fresh airflow.
My parents have one of these in their 3 car garage and in a half hour it heats up the garage nicely.
http://www.northerntool.com/shop/tools/product_595_595
Id get one of these and then just section off one of the stalls with plastic and work in there.