I am at my wits end.

When you installed the new pump, did you use a gasket? Small blocks don't use one. All the small blocks I've rebuilt had the front plugs installed.
Could it be a faulty oil filter, try a different one and see what happens. Are all the gaskets installed in the angled filter adapter if you're using one.
Take a lifter out and spin it by hand and see how much oil comes out of the lifter galley. I would be worried that there isn't a bunch of oil coming out of the oil pressure port when you spin it with a drill.
The cam bearings could be out of alignment with the cam tunnel feed holes, but you said you checked that.