Odd fuel problem

Carburetor icing occurs at air-temps below about 60*F and under high humidity conditions; ie this time of year. And is usually worst while cruising with the butterflies nearly closed.Atmospheric moisture freezes on the edge of the butterflies and chokes the venturies down. Slapping the gas-pedal breaks it off.But it reforms almost instantly. The usual cure is to provide heat to the underside of the carb or to the air entering the carb.It is very hard to diagnose on account of by the time you get stopped, and look, the ice has melted.
Once the intake manifold has warmed up, and the carb on top of it;then usually the problem goes away.
Too much ignition timing will cause the butterflies to be open just a very small amount at steady speed cruising,exacerbating the problem.
As to running lean;
Cold engines like extra fuel while cruising,on account of some of that low-speed fuel will condense on the intake floor, the port walls, and even inside the chambers, passing through the engine unburned.It is the job of the choke to enrich the mixture until this phenomenon stops.
Perhaps your choke needs to stay on a little longer?
Or perhaps the fuel level could be a tad higher?
Factory heads and intakes have an exhaust-crossover passage under the carb to heat it up quickly.A heat-riser valve in the exhaust forces some of the exhaust to flow from one side of the engine to the other,through this passage.
Of course with headers, the exhaust in this passage is more or less stagnant. Air-gap type manifolds are designed to run cold.