12-14psi at idle would seem to be oil pump if the motor was freshly rebuilt. Being a crate motor, this should be the case. Anyway, if the oil pressure guage is connected immediatly after the oil pump, that says pickup issues. Especially if it starts higher and falls low. If it stays flat @ 14 and doesnt rise or fall, thats usually the pump. Or the drive gear. Either way, that motor should be warenteed. Send it back.