Blown headgasket?

[tsubaki]

That's a long way thru corrosion to get to the threads.
A combination of heat, impact driver, even trying to soak the threads will make you cuss.
Chances are, they are gonna wring off about 1/4" from the head. At least you can tap the housing and remove it to spray and heat the bolts.
An impact driver has been effective about 25% of the time also.
http://www.harborfreight.com/cpi/cta...emnumber=93481