I agree completely. I'm far from perfect and could easily have reversed something. Right now all I can do is go over all my pictures I took while I was doing the build. A little later I'll be able to put my hands back on the motor.
All I had the time for was to spin the oil pump tool a little more. She's at 10 psi, but there's no way in hell I'm going to leave it like that. It should be at 60 or so. A little later I'll go through the hoses and if I don't see something I'll pull the remote filter adapter and go from there.

Oil press relieve valve/s?

Hole drilled thru galley plug looks a little big to me too. I drilled the same hole on my 383 Camaro eng. Don't remember the size but it was definitely smaller.

Are you talking about the oil bypass valves or are the relief valves something else you're talking about? The hole is a little bigger than I normally drill but I don't believe it's 6 psi oil pressure big.
I haven't been able to get back to it yet. It's in my yard on a trailer, but I wasn't able to get back to it last night. Later today hopefully I'm going to first reverse the remote oil lines and test it with my priming drill and if that doesn't work I'm going to try putting a filter directly to the block and eliminate the cooler and the remote filter lines. They're the original lines when I got the boat and the motor was a mess. I should have looked at them closer to confirm they were ok.

I didn't think about the hoses going to the oil cooler. Ive seen the inner lining on some of the hoses collapse on the inside but the outside looked good.

That's a good point. I was thinking they might just have crap oil plugging them, but you're right. They could have delaminated on the inside. This is what the oil pickup looked like when I tore the engine down. She was ugly... PO had no clue how to maintain a motor obviously. Oil change? What's that?

You are using the old lines and cooler?
Did you clean them at least.

Yes. Using the old lines and cooler. The cooler was basically new so I just cleaned it. The PO did throw a bunch of new parts at it. Old lines are the unknown right now and could be the issue. I'm hopeful...

You spend thousands on a rebuild and not $300 on new cooler?
Was the engine blown or just tired?
Did you pressure test the cooler? If it has a leak between the oil and water internally you will have low oil pressure also.
You cannot clean all of the metal from the oil section of these coolers, if the engine was blown.
Good Luck I hope you find it and it is something simple.

It wasn't blown. Just neglected, tired and docked in high salinity salt water.
Well, that sucks. I pulled the remote filter adapter and threw a filter directly on the block. Same result. Must be internal. Lame.

Score! I just figured it out. I was studying the pictures of the last 454 I built and this one. There are three oil galley plugs in the valley I left out. Pain in the ass, but could have been a lot worse. I have to pull the intake. You can see them next to the spider bosses in this pic of my other motor.