I had a 69 Dart Slant Six for 19 years. For 18 years it always idled rough and often died if I didn't keep tapping it at stop lights. This was thru several valve jobs, 1 engine rebuild, a new long block, at least 2 carbs (Holley 1920), and Crane Cams XR700 electronic ignition. I suspected poor venting in the tank or boiling in the supply lines. After it still idle bad after the long block, I tried a 3rd (or 4th) carb. It then idled extremely smooth and rev'ed on command. No more stumbling or delays. This suggests there are a lot of bad rebuilt 1920's, or maybe they have a design defect. The good carb had a hole drilled from outside into the barrel. I thought a mistake, but maybe that was the solution.
Another subtle thing is distributor phasing, meaning insure the rotor aligns with the tower when it sparks. Usually designed-in with points, but some people don't think of phasing when they add electronic pickups. The way to check is drill a hole in a spare cap at the #1 and look at the rotor with a timing light. It should align, with slight lead with engine speed and vacuum. If in between 2 towers, it can spark 2 cylinders at once.