How can you heat bolts with a MIG welder? I've used an acetyline torch, and even a little Propane torch on small areas, but I can't imagine how a wire welder would do any heating without piling up a huge mess of metal in the process. I'd suggest giving the bolts a good warm-up with a torch, and a liberal squirt of Kroil, then heating again after a few days before trying to turn them.
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!