question on 302 heads for my sons 318

1/4 inch coarse thread tap and 1/4" coarse thread allen head set screws with a little loctite on them, and you are good to go. Depth isnt that important, just deep enough so the set screws are below the manifold flange. Take a machinists file and dress the area around the plug as well, the metal pulls up a little when you tap.