Had many Subarus over the years and, generically, my original screen name was Subaru_dude. Have no idea if it still exists. Still got a gc8 Impreza though, and so does my aunt.
It is in car, radiator was removed for the head gaskets and hasn't gone back in yet. A machine shop loaned me a time sert set to try out with a liberal application of loctite 271. If it doesn't work, they said it will need to go to them for repair.
You mentioned you're doing it in car right? At this point, might as well remove the radiator to get the best possible outcome for tapping a hole... especially if it might break through into block.