Mike hit it right on the head, the pump is NOT self priming. You need to take the cover off again and pack the entire gear housing solid with vaseline.
I like to take a valve cover off so that I can physically see the oil coming out of the pushrods and into the heads, which lets you know that the whole system has oil, the lifters are fully pumped up and the rocker shafts are full. The clattering noise is another good indicator, since that goes away only when the lifters are pressurized and thus closing the gaps between the pushrods and rocker arms.
It's a situation unique to engines based on the Buick 215 design with the front mounted oil pump. Jeep V-8's have the same situation too, since they are actually based on the Buick as well.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.