The issue we face when removing a manifold is compounded by the stud extending into the water jacket. When the nut refuses to budge torque is transferred to the stud which cannot turn because the end that protrudes into the water jacket has enlarged due to corrosion and is now wedged against the crankcase. So we apply some more torque and the stud breaks. Now we get to drill out the stud, clean up the threads and install a new stud. If we are really sharp, we will carefully install the new stud only far enough to prevent the end from extending into the water jacket and the corrosion problem rearing its ugly head for the next guy who removes the manifold.
The bolts securing the two manifolds together on my car were so degraded that just the slightest torque snapped them. Fortunately there was enough of a stub sticking out of the intake manifold to allow grabbing the stub with a stud extractor and the stub could be unscrewed without damaging the threads. The stud extractor also made short work of removing a couple of studs in the crankcase that needed replacing.
https://www.amazon.com/Shop-Iron-16023-Drive-Extractor/dp/B00265M8N4