EDIT: OOPS I must have been typing while you were typing.... I think 1/8" NPT is what you want. I don't know what the part number would be.
What is 5/8" thread? Water sending units are almost always pipe thread. I have never heard of 5/8 pipe thread, and I can't find any evidence online that it ever existed. I'm not trying to be obtuse here, but I have no idea what sort of threads you are talking about. What was in the box?
