I think you said you changed the bearings? I think you literally have a good tight engine and you have to go to a standad volume Pump. You have too little flow required and the PP bypass cannot handle it. Basic physics. Most race engines in the past used big bearing clearances and big if not huge rod side clearances and needed the high volume PP. You probably have stacked all your clearances to the small side. I sure hope you did not do all that work and put another HV pump in there. Sorry if you did.

So your bearing clearances are nice and tight and possibly the Rod side clearances. Main Rod and rod side clearances affect oil pressure the most. Well rod side clearances could affect pressure if they were way too tight. Be sure and check them.