Heat it up with a torch or even a propane torch if You have one and take Your time heat it up good and hot than give it a good hit with a hammer if that does not work than You may have to grind the thread side down so it's flush and drill it out just start out with a small bit and move up a little at a time when You get close to the size I take and try heating up again and use a punch with a flat tip to try hitting it out again after drilling most bolts come out just take Your time she's been in there a LONG time .
Pete
