I had a guy in town re-build a 350 for me it is a 1976. Everything new finally have it back, took it for a ride and after it warmed up when I come to a stop the oil pressure dropped to about 5-10 pounds. Could that be correct?
Are you sure the gauge is accurate? What kind of gauge is it, mechanical or electric? As a point of reference, the well worn 350 in my '65 had 25 psi at idle.