Well, that was a fun afternoon....
Grabbed all my tuning gear (timing light, feeler guages, distributor springs, vacuum gauge, handheld tach, carb jets) and headed out to the farm roads south of my house...
GEEZ!!!! it was a LOT easier 10 years ago, when all I had to grab was my laptop and my diagnostic port to serial adaptor...
Anyway - Making the move up to the #4 heat range plugs was the right call. The #3 plugs always looked dark (as if things were a bit rich), but after putting over 100 miles on the car with the #4's, these plugs look PERFECT The insulators are the color of pine, and there's a nice clean, gray spark line on the electrodes; and I didn't have to change jets in the carb to get there.
Timing-wise, I left the vacuum advance plugged and focused on that off-idle flat spot. I swapped one of my centrifugal return springs to a lighter one, which allowed my timing curve to ramp up faster off idle and hit the max sooner, and that did the trick... Everything seemed to run best with 14 degrees initial, and 38 total all in by 2800...
I also moved the accelerator pump cam up one notch on the carb primaries, to deliver the primary pump shot faster.
Now it comes away from a stop much better under normal throttle conditions, and the best part is that it's even faster...
No, ACTUALLY the best part was that it never stalled out or refused to re-start. I cycled the ignition probably 20 times today, and it fired every time...