I’ve never had to go into this but here is my favorite trick to remove a stuck bolt. If there is enough access I slip a nut over the bolt and mig weld it on, then a ratchet and socket will usually back it out easily. The big trick here is I try to pull it out hot, don’t put much torque on it hot or as said it’ll twist. Most of the time with the heat of the weld they’ll back out without much torque at all.