Studs suck. Always.. If you can see your way clear to pulling the manifold, heating the studs up and either unscrewing them out or drilling them out do so. Then rethread the holes and put in good grade 8 bolts with neversieze or equivalent. You will never have an issue after that. Studs will always go to pot - metal is lowest grade and do not stand up. Done dozens that way over the years....

trafdlo