Late to this game, but I also drill and use a long tap like Brent. In fact, he drove me to do it this way many years ago. The key though is a good quality tap and it'll cut like butter.
Although it feels spooky as you tap, there is plenty of depth there, you'd have to drive the end of the tap into the lifter port deeply to go too far. I just go a few turns, try the plug, and repeat until it looks good and I slide a distributor housing in to avoid surprises.
I also have not seen any common NPT plug that wouldn't work. There is plenty of depth to clear unless you have some sort of crazy plug I haven't seen.
