Have you pulled the cover off of the oil pump and checked what it looks like inside? I think that you are probably either running into a pump problem or you are missing something somewhere that helps you build oil pressure.
A stock (standard flow) Melling pump moving 10w-30 oil should give a new motor about 60 PSI cold, 45 PSI hot. My 289 is built pretty loose and made 45 PSI hot with a new pump and 10W-30 oil.
For you to go from 60PSI cold to 25PSI hot is an indicator of a problem to me. A new motor with good clearances should make a lot more pressure, and there certainly shouldn't be so much of a differential between cold and hot pressure.
If I had to guess (and this is definitely a guess), as your motor heats up things are expanding marginally and oil is also losing viscosity, so you may be looking for something that is oversized from spec. Essentially, a gap that gets too big when hot, causing a loss in oil pressure and an increase in wear.
Since your cam bearings are all trashed at the bottom of the bearing, I would then further guess that your cam bearings are too loose (oversized), and the timing chain / valve train is providing the pressure to shove the cam down towards the bottom of the motor.
Great post, thank you! The more I have been digging and checking I come back to oiling as the probable cause. I am certain the valve train geometry is OK. The more I look at and research the main and rod bearings and as someone already pointed out here, they might have oil starvation damage as well (but I am still not sure). These are tri-metal Clevite 77 bearings and below quoted is what I found on their site. Given this coating is so thin, and stated for oxidation protection, maybe is normal for it to come off. I am contacting Mahle (Clevite) to see what they say.
"FLASH PLATING: Most tri-metal bearings use what is called “flash plating”. This is an
extremely thin layer (approx. 000030’) applied all over to provide uniform appearance and
protection from rust and oxidation in storage. The common flash plating choices are either an
alloy of lead and tin ranging from 10 to 20% tin content or pure tin. Pure tin has a more whitish
color while the lead-tin alloys are a medium gray color. Both have a satin finish."
The internals of the M68 std Melling oil pump look OK and it seems to function normally. I was starting to suspect lifter bore clearance issues but I am measuring at .002 clearance and the Ford manual states .0005-.002 desired and .005 service limit. I checked this as when I primed with the intake manifold off, I noticed oil coming out the top of the lifter bores. I did not video it unfortunately but it was more than I expected but I have no idea how much to expect.
The lifters I have from Comp Cams came in two different style orifice plate (2 sets as one was defective). The newer model put out way less oil then the older style that I had in there for break in. I am considering if this may be the "leak", but not much out there to confirm how much oil there should be. All my main and rod bearing clearances were in spec based on my machinist's checks, but also my own plastigage check on all main/rod bearings, but all were on the high side of "desired". That said, I did find the plastigage to be reasonably accurate, but I would put it at +/- .0005". Just my opinion.
I am reaching out to my machinist (it has been 3 years since he did the block) to see what he has to say. I'm not trying to place blame, just he has a lot of experience and might have an idea. Down to bare block now, kind of depressing, but we will see how it goes. I am very close to dropping it into the ocean and buying a new motor. I have way more money than time. But I want MY motor!