The size of the plug is 5/8-11. You might need to run a cleaner tap through the holes to get the carbon thats built up out of there. Another option thats simpler is to take that tube you cut off and cut the ends off and then install them backwards with the original bolt to block off the holes. Works in a pinch and nobody ever sees it back there. Good luck with it!
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.
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