Sounds like 20's waaaay too low, maybe for a few seconds, but it should be more like 40lbs, or more.
You should see oil up top pretty quick, maybe 30 seconds?
Don't run it anynmore till you're sure everything's OK.
Pull the distributor, take an old wide bladed screwdriver and cut the handle off. Slip a hunk of vacuum hose over the blade to make a tool to turn the oil pump shaft. Chuck it in a drill -- Clockwise rotation!.
Spin it up till oil comes up top. The pressure should come up too.
If you don't get oil up top even when spinning it with the drill, then pull off a rocker shaft and the stands, see if oil comes out the stand's platform holes now? Oil should just about hit the wall across the room.
It's possible the cam bearings aren't positioned properly, cutting off the oil up top, as that's the feed - sometimes the installer doesn't get the holes lined up.
But the low pressure concerns me too, more than the top oiling problem. Sounds like an internal oil leak.
You could have left out an oil galley plug? Or one popped out?
I assume you checked and double checked main and rod clearances by plastigauge before asswembly?
If still low pressure and no oil up top, it's coming apart anyway. Before pulling it out, try pulling off the front timing cover. Spin the pump with the drill again, just to see what's going on in there. Behind the timing gear I think there's 2 galley plugs. Hopefully you forgot one. The gear and chain get lubed from oil running out the holes from the valley.
Still no, then you can drop the pan, rig a supply of oil to the screen and use the drill again. You'll see where it's pouring out of. It's messy, but has to be done.
Good luck
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