Still hesitates off throttle...

Ryan68Dart said
I had the carb off and checked that the vac advance is plugged into the ported connection. I confirmed that it has zero vacuum at idle with the throttle plate closed then builds to about 18 in/hg as the throttle opens, then backs off to 15 inches at about 2500ish rpms. It stays at 15 inches to full throttle.
I think this may be the problem.

No, I think not—the way you describe is the way it should work.

Ryan68Dart said
The vacuum on that port goes shoots to 12 inches almost immediately

Yup, that's correct. At idle, the spark advance port is just barely above the upper edge of the throttle plate. It takes very little throttle opening to uncover it and expose it to vacuum. That thing about how vacuum advance shouldn't start til 1600-1800 rpm is not correct.

I would return your attention to the carburetor. The formal thing to do would be to double-check that your accelerator pump link is in the correct hole in the throttle lever, and double-check the pump stroke adjustment. For a quick diagnostic check, move the accelerator pump link to the outermost hole in the throttle lever. This will give a longer pump stroke (assuming there's nothing the matter with the accelerator pump system). If that significantly reduces the stumble/hesitation, you know you're on the right track.