As Mark suggested, check valve lash as well as valve lift with another cylinder. Either valve may not be opening enough to charge the cylinder.
I would also suggest putting in a vacuum gauge to see that needle is not fluttering due to valves not opening, should be steady reading.
Could be a lean condition. Isn't this where the vacuum is drawn from for brakes and A/C? Unplug all vacuum hoses and cap off nipples to keep engine by itself. Then go from there, hook one hose at a time until the lean one shows up.
You might even pump alittle shop air into the cylinder @TDC and listen to either the carb, exhaust pipe, or crankcase for leaking air. Compare to a good cylinder. Don't forget the radiator cap to check for leakage there also.
The coil may not be outputting enough to jump the gap for a lean condition. With plug wire min 1/4 inch away from ground, should have a nice blue spark if battery is in good charge.
I'm sure you've swap wires and plugs with a good cylinder to verify their condition.
You mentioned using a scope to look at ignition. Man, I thought nobody knew what these where any more. Hook the scope back up and check the right-hand side of the sweep with all eight cylinders superimposed onto to each other, primary side. At the right side of the sweep is point opening. It should not vary more than 4 degrees, otherwise, the shaft is crooked, or cam is worn. Check to make sure all plug firings on parade postion on the secondary side are of even voltage. This signifies all plugs have the same gap. If one plug is higher than the other, by a large amount, as in off the screen, open secondary side for that cylinder.
All the above assumes you have a point system. I'm sure you've tried another distributor and/or carb.
Let us know what it is........