understanding a low oil pressure 360
What is the mains and rods bearing clearance? Did you prime it with the intake off to look for bleed off at the lifters? What specific solid lifter are you using?
This could very well be the problem.
Get a plan. Check everything possible from easiest to most difficult.......ya know..... try a different gauge, try a different oil filter. I've seen a brand new Napa Gold(Wix) create a similar problem.
Maybe check the right angle adapter thoroughly. Personally, and I don't want to start anything, I don't like them.
Visually check for the aforementioned plugs. If all looks good, pull the intake and see what your lifters are doing while priming with a drill. You MAY just have 16 friggin oil leaks. I know from first hand experience ROLLER lifters can cause that. Maybe the wrong flat tappets can, too.
I guess if all that you can verify is ok, then it's time to pull the pan. I know that's not fun but you cannot run around with that low a pressure.
One thing. From your description it sounds like you're getting an awful lot of oil to the heads.
Sure hope you can get it sorted out. Just be brutal in your endeavor.......FIND that damn problem! You can do it with the help of this forum. I can't tell you how many times over the years this place has saved my butt.
From A to Z someone here is an expert.