I have used a vacuum gauge when setting idle mixture screws. I can 10 out of it which is as good as it gets. It does stay at steady 10 in Park at idle of 1100 RPM. In gear, the RPM will drop to around 850 and then dance around a little from there.

I haven't read this all but will throw out there you may need to drill small holes in the primaries to get it down on the idle circuit. I wouldn't be surprised at 1100 if you were on the main circuit.

Perhaps I have missed it, where is our initial ignition timing? Are throttle butterflies at idle so far open to cause you carb to be running in transition circuit instead of idle circuit. A simple test is to adjust idle mixture screws,,,,does this cause significant difference in idle speed? Crank initial timing up to say 15 to 20 degrees which will likely increase idle speed, then adjust speed back to normal using carb idle speed adjuster, with screw idle mixture screws out from all the way in, out say to 1.5 turns...Any improvement?

Have you tried earlier recommendation to bypass ignition wiring by jumping directly to coil from battery.

Your idle vacuum reading is extremely low unless some crazy camshaft. ,,are you certain of no major vacuum leaks,,,,power brake booster,,,intake to head, cracks in intake manifold. I've even run a garden hose on low pressure on a running engine and been surprised by what I have found.

Any bad cylinders,,,pull plug wires one at a time with engine idling best it can with vacuum gauge connected. Any cylinders with zero change in vacuum reading?

Carb float levels ok?

Initial is at 20. Its a 254 @ 50 solid. Idle screws are 1.5 turns out. That is where is get best vacuum #'s and as steady of an idle as it will get.

I did not do the jump to the coil test. The brake booster is ran of an external pump. No card base leaks or intake leaks using the brake cleaner spray test. No dead cylinders, it runs way too good to have a dead hole. Float levels have been raised to 7/16". Like I said it pulls hard and clean to redline.

The first thing I would have done was throw that POS carb in the trash and put a good Holly on it. I like the ones with the down leg boosters instead of the straight leg boosters.

Meter voltage at plus side of coil when engine is cold, and again when warm. Should be at 12 volts unless you have a ballast resistor,,,about 9 volts then. Does it change between hot and cold, if not I would not suspect battery. Might suspect ballast if you have one,,,or other ignition components, wiring, connectors etc.,,,,hence recommendation to jump from battery plus to coil plus just to eliminate any possibility here. A defective coil could be at fault if begins to fail due to internal heat or as engine warms.

I might measure again when in park and when in drive. Change? If voltage decreases when in gear,,,could be battery if slowed down idle decreases a marginal alternator's output sufficiently.

Can we assume that non idle performance is as you expect,earlier post is a bit murky on this point. If so, I would lean more to a carb problem. Again ask where are butterflies in relation to transition circuit? 2 1/2 turns out on carb idle mixture screws sound excessive,,,what happens when screwed further in,,,,really close in or closed engine should die. If not,,,I am very curious as to butterfly position,,indicating if engine not running on idle circuit of carb at idle but on cruise circuit.

When running engine from fresh cold start and idling at what you assert of is about 1,200 rpm, I assume that you are not running on fast idle cam of carb. If not can you manually force carb butterflies toward closing. Spray a little wd 40 at carb and cable linkage to ensure no friction that may disappear as engine warms. I would likely disconnect accelerator linkage to eliminate this from equation.

I do have a ballast resistor. I even replaced with a spare as it was getting a few years on it. I'll do that test here in a day or two and see what I find. I'll try a spare coil on there too, currently using an MSD blaster.

I'm at 1.5 turns out on the mixture screws. Not running on fast idle cam, I gutted the choke mechanism quite a while back. The linkage is free and the throttle shafts are also good. My guess is that its right on the edge of idle/main circuit given the idle rpm. Again, in gear it fluctuates around 800 or so. The battery is new and the alternator tested fine also.

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.

I would

The convertor is a 9.5 PTC that was spec'd for car, cam, gear, etc. It has never had a good idle to be honest. Its been in there as long as I have had this motor combo together. At that time I did have the screws out 2 turns but have since went back to the 1.5 setting. I'm going to see where I can get some race gas and try that next along with electrical checks. Thanks, I appreciate the help.