Coming to the party late here, but I had a similar problem a while back - ended up tracing it back to spark plugs - short reach plugs in long plug hole, probably were sparking outside the chamber

I also had to set my initial timing way high like 16 or 18 degrees and the truck would not idle well below 1000 rpm but it made great driving power

Once I changed the plugs everything else fell back into place and I undid all the "fixes" I had previously attempted - now we all cool and all is at it should be. Don't overlook the little things and make sure your plug gaps are correct for your build and the same on all cyls.

Good catch on the bad vac adv on the dizzy