1/4" tap works well, the smog ports are almost exactly the right size already. And the set screw only needs to be a 1/4" long, so you really don't have to cut many threads into the head, just enough to get the set screw to sit flush with the outer edge of the head. A little loctite and you're good to go.
The ports are tiny so it's not as much of an issue, but welding cast iron without pre-heating and an extended, slow cooling period isn't recommended.