I'm assuming your heads are emissions rated. You'll find one hole drilled and tapped at each end of your cylinder head. If you're not using any emission/thermactor hookups you should plug the hole at the back of each cylinder head. In an emissions-exempt vehicle, the rear holes may have been left open or unplugged by mistake because the person simply didn't know it was an exhaust leak. To plug the holes, you can visit your local bolt supplier and get 5/8-11 x 3/4 stainless steel bolts and apply a good thread sealer because the bolts are pretty coarse.
I also believe you can buy the plugs that Ford uses but I don't have the part number for those at the moment.........Found it: OEM Ford 5/8-inch blind hole insert (Ford PN 351418-S).
Good luck
Michael Plummer