Personally, and given my recent head gasket jobs I'd whip the head off and do it on a pillar drill, rather than attempt to drill it out with a right-angled adapter as I know other have managed int he past. Your head will need ot be faced before replacement and any decent machine shop doing this will be abel tot ake the stud out for you anyway. We have an excellent shop who did Steve's heads recently. They came back looking brand new.
Martin