Broken bolt - thermostat housing

[LShute]

I have to replace my thermostat (90 diesel), and it sure feels like I'm going to snap two of the bolts off that hold the housing to the head. I've been thinking about this for several days, planning to put a torch on the head as close as I can to where the bolts go in, and try heat. But lots of bad visions dance in my head. Cracked head, warped head, ruined gasket since the head will be getting real hot but not the block. I'm wondering if it might be best to just let the bolts twist off, if that's what they do, then try to drill them out. I've got some PB Blaster (heard about it by searching this site), but I can't get it anywhere near where it needs to be unless the housing is off. If I twist the bolts off, what next? Thought I'd soak the holes, or stubbs if any are sticking out, with PB Blaster, then try reverse drills and hope I can drill the bolts without screwing up the female threads. If I do screw up the threads, drill them out a little bigger, tap for a larger bolt and go that route. But, the problem now is whether to use heat on the head. The more I think about it the worse that idea seems. Thanks for any help.

Larry