This is dumb, but I have to ask. I had low pressure from a "rebuilt " but never fired engine once. The oil pan was mashed a tiny bit from where he had been setting it down in the shop. I pulled the engine back out and did what you are doing... I pulled the pan off and found the oil pump pickup had made a mark in the pan. It was blocked at the bottom. I put a better oil pump in, flattened out the pan and was money. I don't know what pickup he used but it was way too close to the bottom of the pan with the shroud missing.... It was sealed up on the pan. I chased that one for a week. Another thing, was it really rebuilt? I'd be pulling the pan and checking the screen.