318 exhaust manifold bolts and nuts....

The studs are course in the block and fine on the manifold side, but that is not a big deal. The proper end stud has a little bump in the middle so it seats in the coarse hole and you should use sealer on the head portion. They stud them as they are not in a blind hole, but wet so you dont leak when you remove the manifold (and possible because they know they will break if removed after 20 years of heat cycling and corrosion on the head end). Im about 50% successful in taking these studs out with the lock nut method as you used. I use heat and ATF/kerosene to break bolts but I still end up twisting 1/2 these right off. If you do break one in the head, all is not lost. CENTERPUNCH the briken stud (very important) and use the smallest drill you can find to drill a hole right down the center. I use a carbide bit for PC boards (HF sells them in a grab bag box for like 3 bucks, they are TINY) and then just start using larger drills in steps to enlarge that hole. Once it gets close to the tap size and very thin, you can sometimes take a punch and dislodge a side off the threads and then use some needlenose pliers and pick it out, or worst case drive it into the water jacket. IF all goes well, you should get it thin enough to start a new 5/16x18 tap (F drill .257, or a 1/4 .250) If you flub it up and the coarse doesnt grab, you can go up to the 5/16x24 size I drill @ .272 an tap.