electronic smith oil pressure gauge

[spdbrake]

I assume you are using the the Smiths pressure sender and not the Autometer sender that came with your factory gauges?
https://www.petrolheadheaven.com.au/...ra-gauges.aspx

If so there should be only one wire going to the sender and its not a floating ground type so needs no strapping to ground. The NPT fitting should handle that (unless you have a lot of Teflon tape on it, actually you shouldn't use any).
I'd start by using a meter on the SMITHS sender and reading the OHMs from the spade connector to case. It should read one of the range specs for your particular sender. EX: 33 ohm engine off, 240 ohm full sweep. (Those are VDO numbers I'm throwing out)
Also disconnecting the the sender the gauge should peg at full sweep.