Actually those engines run super hot EGT ,thats why 80 percent of them have that failure,the manifolds warp and snap the bolts off........I have one that has done the exact same thing 3 times.......Pat has you covered, the solution is that they tap the hole deeper into the water jacket,you now need sealant on that bolt .

A permanent cure is to use headers so the manifolds wont warp and snap the bolts,very common problem with these engines and the after markett manifolds warp way worse than factory iron.