[Busted Knuckles]

As many rebuilds as I have been part of, we've never had problems with worn lifter bores, at least not enough to cause oil pressure issues. They stay bathed in oil and have little side pressure, even big blocks.
I'd double check for restrictors. If everything else is correctly aligned and the passages are clean, plenty of oil to the bottom end...pull lifter gallery plugs and make sure the oil bands on the lifters are where they should be...saw this as a problem once on a late model block with a cam that lifted the oil band completely out of the gallery. I know it's a longshot, but what the hey...

 

[Busted Knuckles]

Engine flushes aren't much more than concentrated solvents. If you use one, don't run it too high revs and don't load it, that's the advice I got from my guru. Get it up to temp and let it fast idle and drain it hot. They reduce your oil's ability to form and keep a film, so you have zilch bearing protection at higher revs and under load.
I don't put a lot of stock in 'em. If it's sludged up bad, they won't touch it.

 

[Busted Knuckles]

You're right on the analysis of what my guy said, high idle no load at temp for a few minutes.

Lifter bores can be a fraction of a degree off when initially machined and there are understandably variations from block to block. That's why I've always heard not to re-use unless they're back in the same spot.
HOWEVER, since all your lifter bores should be on identical angles and the lifters were broken in on those angles, I'd try it. Use some of the break-in lube, shouldn't wash off with just a prime...

 

[Busted Knuckles]

With the filter acting stupid, I'd swap the spin-on adapter since it's involved in the circuit...