Ford parts sells a plug for this hole, that looks like a threaded insert. It is used in the front of the head for accessory mounting, (alt., p/s, etc). Don't have the part # handy, but I can get it, if you want. A short 5/8 n/c bolt will work, but then the bolt head is showing. Might be a 3/4 n/c bolt thread, I'll have to look.
