LOW (<6psi) oil pressure on fresh rebuild. Help!

[BerniniCaCO3]

I own a 1990 colony park wagon (panther body) and as part of a community college class, rebuilt the 5.0L for it.
If I was going to go through the trouble, I decided I'd improve, so it has a general conversion to the mustang/lincoln HO setup, with the computer and different timing order to match.

I didn't finish installation at school, so I buttoned it up at home in the garage and started 'er up. I also, btw, installed a suite of sunpro gauges to go with it. A colony park has idiot lights instead of gauges for most things.

My oil idiot light comes on at idle, goes off with gas if I rev it past 3k.
The idiot light is a switch set to 6psi by the way.
The sunpro electric gauge reads 0psi at all times. It also, however, is only being fed with 10V, not 14V (I guess I chose to tap into a hot feed that has too much resistance in it, will look into that another day). Being just a variable resistor that grounds through the oil pressure sender, if the hot is 4V low to begin with and if pressure is never much, it would read 0psi. Once I get it running right, it might say 30 instead of 40.

It quiets down after a couple minutes of running. Not good quiet, just quieter, as if the lifters are starting to pump up with some pressure to reduce the top end noise.
The oil filter is full of oil too (had to pull it off to install the sunpro oil psi sender, after having run the engine).
I'm hoping to god that between the oil filter having oil, and my idiot light, that this engine had SOME oil pressure and not NONE, as that would make the difference between being quite salvageable, and having trashed a fresh and expensive rebuild?
Not trusting the sunpro electric gauge being fed with 10 volts, I might see if I can borrow and hook up a true mechanical gauge tomorrow, for what it will be worth. And pull a valve cover to see if anything is making it to the top end, at all.

 

[BerniniCaCO3]

Alright, now for some other details of relevance.
I'd like to avoid tearing apart the engine chasing guesses if possible, and go after the most probable thing first.


I put in all new bearings, checked clearances with plastigauge and they were tight, and made sure to align all oil holes in the bearings.
Loose bearing clearances explain low oil pressure in old and worn out engines,
For my dangerously low oil pressure, unless bizarrely I spun a bearing without ever putting a fresh engine into gear, these won't be the cause for me.

I hot tanked the block, several times, and replaced all of the oil galley plugs, front, back, and the little one on top.
I installed the threaded ones with pipe thread sealant, and the brass tap-in plugs with gasket sealant, as I was instructed to do.
The tap in ones, I hammered in with modest force just until they bottomed out with a *ping* against the block.
I called my professor today to double-check, he said that was the right way... and it should be unlikely that one popped out.
Obviously, if one did pop out, that would explain it Hoping not. It'll be 8 hours to get to them and put it all back together again.

I did prime the pump: method was that before the block was all the way together and installed, we had it upside down on the engine stand, poured oil in the pickup, spun the pump a couple times by hand until oil came out the other side, called it primed. Did buy a brand new standard-flow melling pump.
Should I have done differently?

Other speculations...
I've heard that the pump has a relief valve, could it be stuck open, bad even brand new? And melling at that?
I've also heard (as a suggestion on an old thread elsewhere) that the pickup screen --also brand new-- could have come out of the box set too high up, and be sucking air? Ever heard of that actually happening?

I also just read this:
"It is quite possible that the lifters are installed 180 degrees off, which allows the oil to bypass the lifter"
True? I put in somewhat cheap chinese (CAT) lifters. I didn't know they were chinese until they arrived in the mail... guess there's a reason they were on "clearance" from list price $200 down to just $60.
But can lifters be installed around the other way in these engines? I know I paid no attention, they looked symmetrical. And can bad lifters explain such very low oil pressure? I don't mind getting better lifters than CAT, but if lifters are unable to chop oil pressure down to 6psi @2500rpm (around where the light starts to flicker off), then I won't waste my time tearing off the upper intake.