For nasty bolts and screws, I first do the kroil thing and give it some time to work. If not I'll try some heat, the heat usually always works. Or you have to grind and drill. If its a stripped out hole for a sheet metal screw and you want it factory stock, you would need to weld it and redrill it. Another nonstock option is, pop rivet nuts, you can get em at harbor freight too.
http://www.threadrite.com/pop_nuts/
