See you are getting lots of advise, thought I'd through my 2 cents worth in. I got some good information from a post I did about same subject:
penetrating oil a couple of doses starting a couple of days before you actually try to remove it; and heat. I used a propane torch on the area AROUND the bolt; not on it. That expanded the surrounding metal and broke the rust free. I couldn't get a drill to reach my bolt, so I used a dremel tool equiped with a small diameter grinding wheel. cut just enough slot to fit the biggest screwdriver for the bolt size. Happy to say it worked like a champ.
