I am at my wits end.

What about the oil galley plug at the rear of the block that is INSIDE the block. You must remove one of the plugs on the back in the bellhousing area to access it. It is to the driver's side of the distributor and plugs the driver's side oil galley. There is also a plug that installs up inside the block where the rear main installs. If either of these plugs is left out, you will have zero oil pressure.