I have remove many bolts by welding a nut on just did the same process on my 5.3 all I use is a little cheap 110 flux cord welder and a nut with an opening about the size of the bolt. Crank up the heat and weld away fill the nut then grab a wrench right away and slowly turn back and forth. if it moves even a little shoot a little on blast in there and the heat draws it into the threads works like a champ.