If you have tried prying the bolt up while turning it, then you have enough room to pry it up and cut the head off of the bolt with a cutoff tool. I'll assume that you don't know what that is, and if you do I apologize for my assumption. Its a 3 inch diameter disc that is put in an air driven die grinder. At high speed it'll cut through steel very quickly. We use them in our auto shop all of the time to cut out fender panels, or the heads of bolts that are corroded on. If you don't have access to one you could use a Dremmel tool with the one inch diameter cutoff discs. They'll allow you to get into tighter places but the discs are very fragile. If you don't have access to either, your best bet is to just take the car to a body shop and ask them to cut the head off the bolt. Good Luck!
