If the old oil pump seized, did you also replace the intermediate shaft that drives it? Otherwise, the hex end might be stripped to the point that it slips in the oil pump, and you are not getting full pressure. Other alternative is, your bearings are smoked, and its time for a rebuild. You might be able to get away with just redoing the bottom end. (may not even have to pull the motor....) so long as the crank or rods aren't screwed up.