Misfire at cruise, popping/afterfire at 3000RPM

1 st Put your hand over the tailpipe and see if it sucks in pulse on your hand. Exhaust valve/s may be hung open... otherwise it night be ignition related...but sounds like exhaust valves are open , why is the thing to find out.
2nd, check carb for fuel dumping through mains.
3rd Leakdown test it, or try pulling valve covers and tapping the exhaust rockers with a rubber mallet.
Report back.
I tried feeling for a suck at the exhaust, but it was hard to tell. All I have right now is open headers, and the exhaust was too hot to get my hand close enough to feel anything other than the outward pulse. Center carb dumps plenty of gas when I hit the throttle, nothing excessive at idle. Starting a compression test now, followed by a leakdown test. Will come back with some numbers.

Does it do it with the vacuum advance disconnected?
Yes it does, no change with the the vacuum advance disconnected.