That plug

Can be a real drag to get tapped deep enough. Since it is captive behind the distributor I often just leave it as a core plug - it can't "pop" with the distributor in the way. If you decide to tap it make sure the new plug goes all the way in - test fit a distributor in there before starting assembly. I often have to take the NPT plug and trim the face back in a lathe on OE, Genesis and Pond blocks to get adequate clearance.

Barry Rabotnick
Survivalmotorsports.com