5/8" coarse thread. I got set screws for mine. your ford dealership might have them in stock, or they can order them, too. you can also cut the crossover tube off the part that attaches to the head and flip those over and it'll work just the same. good luck
Hey when I took mine off i was looking for a bolt too, while I was lookin around my friend was being goofy and threaded a spark plug in the hole. And amaizingly it fit! So i took the plug back out and took it to the hardware store and found a bolt that had the same thread. So just find you a old plug out of your car and do what i did. Sorry so long. good luck!
The Ford part number is F4ZZ-6EO86-A and 4 of them cost me $10.96 at the local dealer. As others said, you can just buy the right size short bolt and put some thread sealer on them. If you have a hack saw, you can simply cut the ends off of your crossover pipe, and flip them over so the flat-back of the fitting covers/seals the hole, and bolt them back on. That's a freebie approach - be sure to use some high temp silicone to get a good seal.
Just take off the smog pump, disconnect all the hoses and junk, use shorter belt (do a search to find the correct length if it will work for ya)...there is a smog pump pipe that goes into the back of the two heads...you can leave it there or remove it but plugging the holes would be best...
To plug them up...
They sell thermactor plugs which "plug" up the hole where the smog equipment goes...you can get them at a Ford Dealership - part # F4ZZ-6E086-A
and you can buy some from Summitracing - part # TFS-51400265
They screw right in...they cost anywhere from $2-$8 bucks depending on where you get them from...