440 six pack high idle

I actually chased a very similar problem with my six pack. In my case the major culprit was actually the distributor. A pair of weak advance springs were not drawing the mechanical advance back down and when I was timing it I noticed it. Eventually it would settle down and the idle would come back down. I wish sure it was some kind of vacuum leak and did all of the standard tests to no prevail. The distributor had a couple small other issues so I just replaced it now my idle is a beautiful thing. Most likely not your problem but something you may look into.