there are lots of things that will effect oil pressure on builds even with the right clearances and oil pump/filter/pickup/pan choices---

besides the choice of what lifter, rockers and up top stuff---distributor fit in the hole(o-rings around base) loose or missing or too large a bleed hole in the front galley plugs--
champered cranf oil holes that are misplaced

oil picpup that has fallen out of the pump, pick up against bottom of pan, pressure relief valve backwards, etc

IF YOU PULL THE PAN---check each and every bearing including the thrust ----

if your brave---with the oil pump removed you can pressurize the oil gallies with compressed air and see if you notice an area where there is unusual differance in the blow out

good luck and hopefully something simple($$$$imple)