I don't know Olddog, in this case he has potentially a lot of thread left at the bottom of the hole, adding the studs and using all the existing thread should do the trick. Of course I'd repair the hole in question and inspect the others.
My concern with something as important as the block would be to do as little as possible to preserve what I got. There is always the potential for things to get ugly if you heli coil enough of those holes, binding, drilling off just a bit, helicoil sticks and won't thread correctly into the new hole. Maybe I'm just paranoid, but I've had a couple of heli coil installation jobs that took a left turn and ended up being a real nightmare. Less is better in my book, which is why I would probably try a chemical fix myself as the first step. Thats a hard call though, yeah, I might go helicoil first time in this case (for ONE hole).
