heat & wax
Some wise person here told me about this method when I was trying to remove an exhaust stud....WORKED GREAT!
Just heat up the bolt and then hold a small birthday candle to the area where the bolt goes into the threads...the wax creeps in I guess and works wonders!
I had also used PB blaster, so not sure how much that helped, but it wasnt BUDGING before the hot wax treatment.