[eagle]

First I'd use a fine point acetylene torch and heat the stud red, then cool with wet rag. Do this a few times then drill out. Use a very small drill bit first, run drill slowly and make sure its centered. Then progressively use larger bits. I'd use left hand bits, one of them may end up turning out the stud. When done use a tap to clean out the threads. May be time to heat/cool the rest of the studs and replace them all. Be careful to heat the stud, limit the heat on the surrounding cast iron block. The heat/cool treatment will pretty much loosen even the worst offender, just keep at it.