TDC is confirmed. Piston at top and mark is correct

This would be a good point to be sure the egr valve is not leaking. A removal and cleaning the carbon off the surfaces may help a lot. Over the years, faulty egr valves have caused a lot of stumbles and hesitations. If they are leaking exhaust into the intake when its not suppose to be is a pretty regular occurrence. Worn stems in the egr valve tend to cause a vacuum leak as well.

We are all assuming you have inspected all the vacuum hoses to be sure none of them are faulty, correct?

As far as my help, its been 30+ years for both me and 50 years of wear for the car, since I've dealt with this stuff. This thread is like going down memory lane for me.

The ported outlet should have no vacuum at idle and should increase as the throttle is slowly opened.

Did you happen to verify that the timing is returning to its initial setting at idle and not hanging up or "floating" down?

Also verify Total timing.

Based upon n your statement about the ported vacuum the vacuum advance is already hooked to a port like that.

That's good, disconnect + plug the port. See if the 60 mph surging goes away

FYI this is on hold for now. It drives better still stumbles and has a lesser surge with the EGR unplugged and capped. My Demon needs to get buttoned up for dragweek and this isn't a priority until then.