If you drill it exactly through the center at 5/16 or 19/64 plus a little bit of luck you will be able to pick out the stud remnants with a center punch, needle nose pliers, etc. Then all you have to do is run a tap in to clean up the original threads and you are good to go.
The closer to center you drill the better your odds of saving the original threads.
The problem is if you drill off center now you have a hole that is partially cast iron manifold and partially steel stud. You cannot tap threads into such a hole without breaking something. In fact, you will probably have trouble even drilling it because the iron is softer and the drill will go off center and jam. At this point you will need to pick out the steel remnants, then drill and tap for a helical thread insert.
If you really screw it up then it is possible to weld up the hole with nickle rod. But before you do that you are probably better off finding another manifold, even one with broken studs so you can practice drilling on center!
Good luck!
