Soak it with a REAL GOOD penetrating oil for a couple of days. Then try to get a pair of vise grip type pliers on the top part where it comes through the flange and turn it through. Depending on the location you may still have to remove the manifold from the engine. Try to drill through the center (center punch first) as large a hole as you can without getting into the flange threads, this will "relax" the bolt in the hole a little bit.
If that fails, just drill the thing out and use a grade 8 bolt and nut and go have a beer, or two. Good luck Turbota.
