I posted in Diesel Section, but I'll post here as well.
You could JB weld a Allen Head wrench to the bolt.
Or you could just use some Vice grip pliers and try turning the bolt. That's what I had to do on my old VW. I had over tightened the flywheel bolts, so when I turn the car on the flywheel broke free. I had to go back in and take out the remains of the bolts using Vice Grips. It was a pain in the butt, but I finally got them out.