Is it a Shelby Block? If so the drain back holes in the rear main bearing cap are not big enough. The cap has three holes that go from the crank to a milled slot. The holes are too small and the milled slot is often filled with your pan gasket. If you have a windage tray check that the tray and the gaskets are not obstructing the
oil return. I fought this problem for years before I opened the holes to a slot and deepened the milled passage. I also notched my windage tray to help the
oil return. If you have a Shelby block, drop the pan and look at the rear main. It will suddenly be obvious.