Jerry's advise is generally the text book guide to selecting power valves. Not sure what happens if a lower range one is used than needed - possibly a rich condition.
Are you carbs sensitive to idle mixture adjustments and the primary throttle blades resting around the middle area of the little idle transition slots in the bores? Or are they resting at the top of the slots at idle?
And I assume you are tuning the idle mixture and speed on both carbs equally?
Normally I would think you are just fighting the characteristics of a fairly big cam if those duration figures are at .050 inches lift, but if you can pull 12 inches of vacuum at idle I think you should be able to get an outcome that isn't obviously rich. But even my 231/236 @ .050 cam with my dual quads is not something I let idle inside my garage for long.