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

As previously stated, the tap used was probably a 3/8 - 24 [national fine] thread. The hole in the block was tapped 1/8 - 27 NPT originally. I'd remove the goop and thread in a 3/8 - 24 bolt. If it threads into the block 1/2'' deep or more that's good. Buy a short bolt of the same thread and a copper brake washer. Shorten the bolt if necessary. Drill a 1/8'' hole thru it. Weld a 1/8 NPT coupling or bushing to the hex end. Pressure test it. Screw it into the block using the copper washer as a seal. Add a sending unit.
good luck