Another 302 head question

I'm presently port matching set of 302 heads to install on my early 80s 318. I read sometime back that smog ports on 302s have to be drilled and tapped for a 1/4 20 set screw.
quote:
The only bad thing about the "...302"s are that they have smog ports, so that you need to drill ( slightly ) and tap with a 1/4 20 and screw in some allen set-screws to plug them up.

End quote

My question is, has anyone done this and how far in do you have to drill and tap for the set screw plug? Is there anything else to this? Appreciate any help. Thanks Mike