Still haven't figured out the idle issue...

Stock idle settings for the Dart Lite setup....whatever that was! Maybe 5 degrees BTDC? I never did anything to that car except put in gas and fix what broke! I would also put it in a lower gear going down a mountain and see how high could get the vacuum readings to go with the throttle closed.....27" was not uncommon. So part of the 20" idle vacuum for the OP may be just due to the high idle RPM.

My '62 /6 with a mild torque cam and an .050" head shave from Doug Dutra idles at 17-18" vacuum with a simple 1920 carb, which is low to me for an almost stock /6.