Thanks for the information. The reason I mentioned "cam" is that it can affecgt the needed air flow at hot base idle, which can relate to how far open the throttle plates are at that time. Usually, the "2bbl cam" from the middle 1960s was the 252/252/.390" cam, whose timing specs debuted on the 1958 350 2x4bbl Sport Fury engine. It was used up through the 1966 model year on 383 2bbls. In '67, the 383 2bbl used the "Standard Cam" 256/260/.425" 4bbl cam (which debuted with the 440/350 in 1966. The '68 383RR cam would be 268/284, which debuted with the 1967 440/375 GTX. So, in looking at the '68 FSM, the hot base idle speed in "N" (per FSM) with the parking brake firmly applied, the idle speed is 650rpm.
As for the idle mixture settings/adjustments, the carb specs list the idle speed screw "Turn Open" to be "1-2". So start with 1.5 turns open to get things going and the idle speed set. Then, adjust ONE screw inward about 1/4 turn and see how the speed might change. Leave it there, then do the same with the other screw. This is leaner in orientation and has NO bearing on how the plugs will color at WOT, per your testing technique.
In the "Lean Best Idle" method, you are looking to lean the mixture until you get to a point (from max rpm at idle) where that "leaner" adjustment resutls in a 20 rpm drop per screw. When that 20 rpm drop, happens, then richen just enough to regain the prior best rpm.
NOW, here's my shadetree method of idle mixture and speed, which I perfected in the earlier 1970s when there was still a bit of lead in the fuel and alcohol-based components were minimized.
With the engine fully warmed up and idling comfortably in "P" after the 20 rpm drops had been noted and compensated for. Nail the parking brake and carefully put the trans in "D", as it would be at a red light. THEN i'd quickly go to the end of the exhaust pipe and check for exhaust gas flow smoothness. I would set the speed just to where the pulses became a smooth flow, by feeling that with the palm of my hand. Being in the exhaust gas flow, I would then smell my hand to check for "hydrocarbon" smell (indicating a bit too rich). Then I'd go back to the carb and lean the mixture 1/8 turn inward, recheck the idle speed gas flow, and then use my other palm to collect the hydrocarbon smell. After each mixture adjustment, you'd need to wait about 30 seconds to do another smell-check AND wash my hands between checks, too. This method seems to work much better on a single exhaust car, but can also work with duals, too.
With the idle mixture and in-gear speed set, then put the trans in "P" and re-check the idle speed. Also turn off the engine and see if it dies immediately. If it does not, then turn the idle speed screw to about 1/4 turn slower. Re-start the engine and cycle repeat for the idle mixture and gas flow smoothness. A "cut and try" process, but when done, everything should be as good as you can get it, regarding idle speed, going into gear nicely with no harsh engagement, smooth and reliable idle speed, and no dieseling when turning off the ignition.
When I was prototyping this procedure, I might consume at least 30 minutes to get it done. Not sure if the smell check will work with unleaded fuels or ethanol variations of it. In some respects, it can be a bit of over-kill past the Lean Best Idle method of adjustment, but using the rpm when the idle pluses become a snooooth flow can still be used, I suspect. In the mean time, that "20 rpm drop" aspect of things can work well by itself. In any event, the ultimate goal is to have the engine idle as lean as it can and still be smooooth at a particular hot base idle rpm. WITHOUT needing an air/fuel ratio meter to do it. Which is what the 1968 FSM says to use, per the new-for-back then underhood Emissions Decal was for (hot base idle speed at a stated air/fuel ratio from the meter).
Back then, too, I had plenty of time and motivation to spend time doing these things. Things have changed a bit since then. So proceed as you might desire.
BEFORE undertaking these things, verify the ignition point dwell (if equipped with ignition points) and then the hot base ignition timing (vac advance unhooked and plugged), in that order.
Have FUN!
CBODY67