Get the screw head red hot with an acetylene torch. It will back out with nothing more than a big screwdriver. Be sure the screwdriver bit fits the entire slot, thickness and width, even if you have to grind it to fit.
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!