Your
oil pressure is way low. Smokey says 10 psi per 1000 rpm. I like to see 50 psi at temp with rpm. Electric gauges suck, get a decent mechanical to verify the pressure readings. Plastigauge is a waste of time. You'll need to measure with tools not a strip of wax. I'd guess that the knocking you hear is the lifters clattering until they fill with
oil. Anyway, if it were my car, I'd pull the motor out before it destroys itself. To take it all apart and check everything will only cost you a set of gaskets, that's cheap piece of mind.