In similar situations, I have used a combination of the suggestions posted here by others. If you have stripped the heads, file two sides flat so you can get a good grip on it with a set of vise-grips. Then, what I would do is heat it up to nearly red hot, being sure to shield anything that may be damaged. If you have a lot of room and the time, you can use a propane torch. Best results are obtained with a small brazing torch set up (O2 and MAPP gas - you can get it at Lowe's or Home Depot): super fast and super hot (note: buy an extra O2 cylinder while you're there - it goes fast). While it's still hot, hit it with some WD40, Kroil, whatever - I use WD40. It will smoke. Keep spraying it in 1 second bursts until it stops smoking. Grab it as tight as you can with the vise-grips, where it takes everything you've got to lock them, and see what you get. They should come right out. Good luck, and please post your outcome with whatever you choose to try.
PS.: The reason I use vise-grips is because they are short. Stay away from breaker bars and big wrenches. The last thing you want is to break off a Grade 5 or better bolt (major project then)
1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick