New oil pressure switch doesn’t fit...318 LA

Is that .395" thread diameter the inside diameter of the threaded hole? Trying to figure out what you tapped it out to....almost sounds like the minor diameter of a 1/2" coarse thread. But that thread pitch is waaaaay off of 1/8" NPT.

If you did tap it out with a straight thread, you ought to be able to find a bolt to go in there so you can figure the threading.