My first step in resolving low oil pressure problems is to swap in a different mechanical gauge.

Cut open the old and new filter to look for metal filings.

There was a common trick to block off oil galleries on Roller and sometimes mechanical cams. Was this done?

If all checks out then pull the pan and double check that the pump isn't chewed up, the pickup tube is tight and free of garbage. Make sure the relief valve is free to move correctly.

If all that checks out then check the crank and bearings for wear while the pan is off.

Hope it helps.