Good luck with the plug. In cases like this JB Weld is your friend.

I don't believe anyone said drill with a 5/16" drill bit. I did say to use a 5/16" tap and drill with the correct size drill bit. This info used to be stamped right onto the tap shank. I only drilled as far as the tap would go in. In your case it illustrates that blocks weren't the only thing to have core shift.

I am convinced that once the AIR hole is cleaned out, filling it with JB Weld would keep things sealed up for the rest of time. So that's what I'd do with the hole in your cylinder head.

R.