+1. The nut is toast anyway, and it is not worth potentially damaging the head or cylinder. Looks pretty easy to get at with die grinder, cut off wheel, 90 degree grinder, maybe even a hack saw (can't see enough in the pics). Just take your time and be careful and patient - it will pay off once you get it apart and only have to buy a new stud for your trouble. I suggest replacing all the other nuts as well when you put it back together to help avoid this situation in the future.