remove exhaust manifold bolts

Buy some plain putty at the hardware store and build a small "well" around each stud. Fill the "well" with what ever solvent you prefer (PB, WD40, etc..) and let it soak for a few more hours (24?).

Peel of the putty and heat the area with a propane torch to warm it up to near operating temp and spray the "stud only" with butane lighter fuel.

This will cause a significant temp difference between the head and the stud breaking most any hold. When it cools soak it again with your solvent and try to move the stud. If it breaks your drillin'