i used to work in a shop where we were either fixing broken manifold bolts regularly or removing the manifolds for headers. soon as i started and they were comfortable with my ability i was given every job like this because it just usually sucks. i also now work in a production facility dealing with alot of heat and anywhere that has a stainless bolt you end up screwed. hardened bolt with some anti seize is all you need. anyway ive used these tactics quite a bit for manifold bolt issues but a few years ago i learned a new one. if your plan is to remove the manifold entirely cut the heads of all the manifold bolts off. one of the big things that binds these bolts is the contraction of the manifold. so cut the heads of the bolts off and pry the manifold off which still wont be easy. however once you get it off now you have solid studs a simple stud extractor will remove or a pair kf vice grips.