I would try the 1/8th npt tap, and when you are tapping down on the bigger hole, dont tap down very far. the tap is tapered, so the farther down you go, the deeper the plug will go when it tightens into the hole. I didn't measure the holes on the last one I did, but it was an L36 lower intake manifold. when I installed the plug in the hole, it wouldn't tightened down all the way and fell into the lim, so I used jb weld, and as far as I can tell, it sealed up. the other thing you can do is the dime mod. it's either a dime or a nickel that should fit into the recessed area of the hole, then just use jb weld to glue it in there. a lot of people have done it like that without problem.
