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.