Sudden Loss of Oil Pressure

I had a voltage problem that I wanted to check if I fixed, so I started the truck it was cold out let it idle to warm up to around 100 degrees, idled it around the block pulled it back into the drive by now engine was around 140 ish. I was sitting in the drive way revving the engine a little trying to adjust the accelerator pump on the FI tech fuel injection system. I just happened to look up and saw the oil pressure go from around 40 at idle and creep down to 0 in a couple of seconds. I shut it down as soon as it hit 0. Started it back up the next day and oil pressure still read 0, I think I gave it enough time for the pressure to come up before I shut it down.

There is less than 250 miles on the 383. The oil pressure gauge is a new Auto Meter mechanical gauge. I have Milodon external oil pickup 21001, I have the correct length oil pump drive shaft. Oil Pump is a high pressure, high volume Pump, I think its Mopar Performance. Camshaft is the smallest Mopar Performance solid lifter cam. Deep sump 8 qt oil pan, and windage tray.

I was thinking something happened with the oil pump driveshaft, so I pulled the oil pump since its easy enough to get to; everything looks ok. I can move the oil pressure relief if I stick something in there to move the piston. How far does the oil pressure relief move? If it was stuck open I should see the top of the piston in the cavity closest to the cap that the oil pressure spring is under?

Since its a solid lifter cam I couldn't tell if I actually lost pressure, its not like the tappets got louder, they can't since they are solid. So it could be the gauge. I know I have a working mechanical gauge on the duster. Its a cheap-o one but I know it works. So I'll pull that off the Duster and put that on the 383 quick to check. I hope that is all it is is just a gauge.