If you just need a plug, you can use a 5/8" stubby bolt and it will go right in.
The inserts which are reducer bushings that go from 5/8 - 14 to 7/16 - 11 so the accessory bracket bolts can screw into them on the front of the motor are Ford part #F4ZZ-6E086-A. They are like $9 for a pair of them. You can also get these same reducer bushing, though they are not a ford part, from Summit...they are a Trick Flow Specialities part.
But if it were me...just get bolts. I"ve also heard some people use spark plugs.
Just a note, not all of them are threaded. In fact I think if you look there is no reason for them to be threaded really. I went to a machine shop and asked them what they do, and he said it depends if they are threaded or not. Well mine were not so he game me the press in plugs, (very small freeze plug looking items). Well I tried to get them to go but no go. I just wanted to let you know so you can make sure they are threaded.
Good luck!
I will probably just cut down some bolts and try that then. I thought they were some oddball thread or something. If their not threaded I guess I can just cut the tube off the little fitting and weld it shut.