I have found a lot of times, it's just the head of a bolt is froze. Especialy sockethead cap screws, If you start with a drill bit just under the size of the allen key (drill bit spins free by hand in the screw) and drill the center at the bottom of the screw head. Have a couple drill bits ready, you'll have the outer case hardening of the bolt to drill through then go just below the depth of the head on the bolt. Some cases the head of the bolt will pop right off, some times not. If not, start stepping up in drill sizes and maybe go a little deeper into the screw head each time you drill. Once all of the screw heads have been removed, the item your trying to remove will (in most cases) will come off with out much fight and damage. You'll then have the shank of the screw left, with an exhaust manifold that could be a couple inches to grip too for removal. I'm sure a lot of us have used this trick. Although while on the road, I have showed this trick, along with the previously mentioned and surpized a few guys on bolt removal without detroying the parts that are held together by them. So maybe worth mentioning.