Idle restrictor started in 1977 it may be dirty or too small for today's fuel I would suggest cleaning it really good enlarging it or remove it altogether. At 5 turns out it's the Restriction that setting in your fuel mixture, not the mixture screw.

Holy great pictures! Thank you so much. I had noticed that restrictor, I see that it's south of the idle transfer slot. Would that restriction potentially cause my drivability problems? Comparing my oe carb to the 7989, the oe is missing that restriction, though the idle mixture screw on oe is capped. Wonder if it was removed, or if the factory omitted it since there is no option for idle mixture adjustment.

well on the contrary, there IS option for idle adjustment on the original carb.... it's just capped with a steel or lead plug in about the same location as your replacement carb....
has a small head that can be set with an allen, once plug is drilled (carefully) out. there was on the orig carb that came off of my 85 D150 anyway (biggest difference between yours and mine, is mine is an auto trans.)
not that it means anything if you are intent on ditching Lean Burn...... which is far and away the BEST place for the LB....
Take it out, set it under the truck and drive over the old computer.

the curb idle thing was least of my concerns.... a piece of bent up flat stock and tapped for a #10-32 and that would have gotten me past that issue.
the lack of a proper vac advance fitting and sloppy throttle shaft bushings was the bigger issue.

Just for Diagnostic and testing purposes, a few strands of wire down the idle air bleed opening, will make the off idle circuit richer. wrap excess wire around the air cleaner support bracket. If the surge goes away that would suggest a problem in the idle circuit. No change that would suggest it's in the main circuit. Reset curb idle mixture after you do this