Are you sure that the oil pressure is after the filter and not before? I don't know ... just curious.
You are obviously doing some good work, and study, as you go on this repair. You are correct in making sure oil flows to all points that it should ...(but how about the crank journals and throws?) ... keep looking. I don't have any oil distribution diagrams for this engine either. Compressed air might be handy for analysis and cleaning. Are you using new bolts each time you remove/install the head? ... getting expensive!
I often use 40w oil ... in fact, I just bought several gallons of Mobil1 0w-40 for hot weather use. I don't think you would see much pressure drop if you changed to 30w ... something else is going on here. Of course, the new pump is part of the issue ... but I would not expect cold oil pressures over 80psi if the arteries are all open. On a well maintained engine, in stop-and-go traffic, summertime with ac on ... I would guess the pressure could easily drop to 20psi, or less, at idle.