Heat is your friend. Get the bushing around the plug as hot as you can with a propane torch, or get it cherry-red with an acetyline rig, and turn the plug out. If a candle or a piece of parrafin sealing wax is applied to the thread area when it's moderately hot- - - -the wax will follow the threads and help lube it, but watch out for a flash fire if you do that.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!