Only way to be sure is stick a gauge on it at the block and see what ya got. I'd be afraid to even idle it very long unless I knew for sure I had pressure.

The pump could be fine. You won't build pressure if a plug got left out and all the oil is just going back into the pan instead of to the crank and the rest of the engine.

I'm trying to remember all the galley plugs. You have the small ones on the outside by your sender, the ones on the ends of the block by the bell housing and timing cover, and I think there are a couple just below the pan rail.

You would have obvious leaks if you left out the external ones but, not if you left out an internal one. The oil would just flow back to pan.

I'd be a nervous wreck until I found out for sure. It would suck to waste a fresh rebuild.