I'd start by pulling the distributor and intermediate shaft. If they look good then spin the pump with hex and drill while checking for pressure.
If none then I'd pull the pump and disassemble. I had a stuck pressure relief valve on a small block pump once so look at that portion carefully.
Do not forget the BB distributors / oil pump drives rotate CCW.
If you cannot get oil pressure, next step is to remove the valve covers and look for the potential problem. IE: broken rocker, other valve train issue that may have allowed a lifter to exit its bore. Next stop, Oil pump itself. DO NOT start the motor until the issue is identified. The drill motor is your friend
