Soak it Real good with wd-40 over night. Soak it a few times. The next day put a nice vice grip on it and you should be able to get it started out. Once out really clean all the holes good. Chase threads, blow out with computer keypad cleaner or compressed air. When reinstalling the headers, its critical to start all bolts just a couple turns, and tighten each bolt just a few turns at a timeline. The header bolts must compress evenly on aluminum heads. Its a pain in the *** because it takes longer, but a must. I've never busted a bolt or cross threaded a head using this method. One other tip is to slightly enlarge the header bolt holes before installing the headers and using lock washers.
Also, spray the header gasket with copper spray gasket maker.
You will drive with leak free confidence when this is all achieved