I built a 350 once with a brand new high volume pump. I put in a 7qt pan. I started the engine, and no pressure. None, zip, nada. I went nuts.
I had to redo the crank and rods and I found the issue. The oil pickup was way too close to the pan bottom. I just put the parts in and turned the key. I never thought to check the pickup length with the pan.
Live and learn.
Two ways I learn, first is to rewrite the book, then pay for the right way. I am getting better.