If you have the oil drained , I think I would be tempted to "flop" the engine over being sure that you are not going damage anything and have enough help.  I would "stuff" some oil soaked rags in the ports or tape over them before drilling to catch all the metal particles. Some of the valves are going to be open and you don`t want that stuff to drop into the clyinders. Makes weird knocking noises and possibly locks up the engine. You need to tune up your eyeball as far as drilling straight. Its really not that hard to do. Just take your time.
   After all this said and done. !m assuming that everything else other than the studs are OK ?