Has car ever run well at idle with current engine/torque converter configuration? If so, when did it cease to do so?

Have you changed to another stall speed convertor? You have indicated that idle is fine in park or neutral, but falls way back when transmission is placed into gear. Any chance that torque converter is too tight for your engine cam/carb/etc configuration? Perhaps forcing you to adjust idle speed higher, thereby placing butterflies at idle past transition zone into cruise where your mixture screws have little consequence.

I had seen earlier post where your mixture screws were more than 2 turns out.