If you have a welder, a trick i use is welding on a piece of bar stock or a nut to the plug. The heat from the weld will often loosen the stuck part. This work on bolts if you have enough sticking out to weld to.

Jeff
1947 Chevy 6400 2-ton
In the Gallery
1953 3/4-ton
In the Gallery
It was fun while it lasted