Chris, your pick up may be sucking air at the case where we use two O rings to seal it. I have run into that before. The other cause could be a restrictive cotton type filter. I use a filter for a 540 Allison trans that would come in a blue bird bus, this uses a screen for a filter medium. Part # FK 135 in the ATC brand. You will need a 400 pan gasket purchased separate. The filter is the exact same as a 400 configuration, but preferred for racing and high oil flow!
Bruce