You need a flank drive socket. Heat the bolt, just the bolt, cherry with MAP, Oxygen/Acetylene or plain old Propane torch. Then let cool for an hour or so. Try a good quality proper size hex (Allen) wrench. Put short piece (12 - 18 inch) 1/4" pipe to use as leverage (cheater pipe) and work back and forth to break tension between bolt threads and any corrosion bonding between the bolt threads and the threaded hole drilled in the head. It should come loose.
If that doesn't work heat the flange (not the bolt) cherry red, and let cool. As the flange expands it will apply pressure to the bolt and its threads, and this should free up any threads that are bound up.
Re-assemble with better quality bolts, use the WHITE high temp Locktight (TM). Any thing I build, which any more is few..., all manifold and header bolts get the high temp Locktight.
A bolt is actually a spring, (spiral incline) which is why proper torque is needed when putting things together so less problems when taking apart. So the threads are in there under spring tension and after multiple heat cycles the "male" threads tend to bond to the "female" threads. So turn it to tighten and then to loosen and repeat, and the areas of bonding should break free.
Cherry Red is high enough temp to get most steels into a plastic state and torque applied too soon will just twist the bolt to separation failure. So you want the heat to expand (stretch) and then relax to original size after cooling. This should be enough to break the threads free and not damage the bolt.