I have a 1965 dodge truck with a 318 with the old canister type oil filter.
The problem is about 200 miles after an oil change the pressure while driving slowly dropped to 5lbs I shut off the engine. When I restarted the engine the pressure went up to 50lbs but starts slowly dropping to 0.

I changed the oil pump, now it holds about 35 lbs of pressure at an idle but if you increase the RPM’s the pressure drops to 15lbs let off the gas and it goes back to 35lbs.

I have also changed the pressure gauge.

Do a mechanical oil pressure test so you can determine if your gauge is acurate.