Rich Idle AFR reading

I read your issues as being the transition circuit not being right or the ignition being weak. No way that you should (or will) have such a big change in AFR going from idle to just off idle'. As said, once it starts to misfire, then the AFR readings are going off kilter; the free oxygen in the exhaust rises and that gets falsely interpreted as a lean condition.

You're having to richen up the idle mixture to cover either a lean misfire or ignition misfire condition just off idle. Working on the advance alone is also just covering up this issue. Find out what is wrong in the carb 1st OR in the ignition energy being produced. If the spark is on the weak side, then it will have a harder time firing through the pressure changes in the chambers. And that can apply to the plugs you are using; we don't know what you have for plugs and wires or how good you ignition really is. Putting in an HEI might be a very good next step for you.

FWIW, if you hit in the 15 AFR range at light to moderate cruise conditions, don't worry over that one little bit. It will be just fine and, with your engine, going into the 16 range at light cruise wouldn't bother me either, my son and I have his high compression 340 in the 15 range all the time on the interstate... not a hint of detonation ever. Fuel economy at cruise will go up quite a bit when you properly abandon the idea of wanting stochiometric AFR under cruise conditions, and it won't hurt your type of engine at all. As a pint in fact, the Lean Burn system pushed cruise AFR up into the 18 range.