We respect you too.... but understand this has been an issue for you for some time, whereas I just heard about it an hour ago, so I, like many others here are simply throwing out ideas, and ruling things out. No offense meant to you.
Well in a dual plane intake like the 2x4, one plane is what, cylinders 1, 4, 6, 7 and one plane is 2, 3, 5, 8, right?
If your issue followed that I would look into a carburetor issue, staggering jetting, making certain no trash is in a bleed or restriction, etc. Or a manifold distribution issue.
So my logic points to things that are specific to one head or bank only. That is why I'd not waste much time on the carb, but would focus more on the sensor (which it appears you have) or the exhaust (which it appears you have).
Now, O2 sensors are funky things. Are they both grounded in the same place? (it matters)
Do the heads have an exhaust crossover that could skew readings?
Are the headers on both banks perfectly sealed?
Are the welds perfect for the o2 bungs?
Was silicone used for the header gasket? if so, is it safe for sensors? (not all are)
How do the plugs look? One cylinder misfiring or one weak plug or wire can toss the readings for the whole bank.
next line of thought.....
If it runs well and none of the cylinders are dangerously lean, well..... rock and roll
Stop looking at the gauges, heh. I know, not totally helpful, but the nature of the beast sometimes.