There are other causes of what you are describing. Bucking at around 2400 RPM is often a fuel issue. On a *warm* motor, it will run normally lean at cruise speed, there is a fine line between efficient and fuel-starved. O2 sensor on a warm motor mostly controls fuel along with the MAP sensor. If your plugs are white after a run up the interstate it is often a sign of lean-burn, if they are sooty it is usually a sign of too rich.
Too rich can be a MAP issue, tiny leak in the vacuum line for the MAP is the more common cause. That line can rub on a wire for years and eventually develop a hairline crack. It also gets very brittle with age and should be handled with caution.
The O2 sensor wiring can be a trouble point. Maybe OHM test your O2 sensor ground. If you really want to check it out disconnect the ECU connector and ohm test the wire from the O2 sensor to the ECU.
Distributor indexing basically adjusts the envelope for the timing, minimum timing and maximum timing. If the adjustment is too far off the ECU loses minimum or maximum timing. The ECU is going to adjust the timing, really no adjustment except the envelope. Timing is regulated by multiple sensors, mostly by TPS.
In an ideal situation retarding the timing will help with the lean burn under load. The ECU controls the timing, no real way to retard timing. The knock sensor is supposed to retard timing when it detects detonation.
It threw me when you said disconnecting the O2 sensor made it worse. In my experience disconnecting the O2 sensor should default the ECU to basic setting, most often a little rich. You might try a drive on the interstate on a cold motor and see what happens. When my Renix was bucking it would happen on the same stretch of interstate and started about 4-5 miles from home, about the time the motor warmed up. One time it was the O2 sensor the other time it was the wiring.
Just a WAG but are you sure the number one plug is at the number one position on the distributor cap?
If I had to venture a guess I'd guess there is something in the O2 sensor wiring. And or multiple little issues that add up to a large problem.
Hook your vacuum gauge up to the intake manifold and see what the needle does, more the motion than the reading. Double-check for vacuum leaks, a lean burn can be a vacuum issue, most likely loose intake bolts. Note; don't try to tighten the end studs on the intake exhaust manifold they are often frozen and weak from rust etc. and very rarely the source of a leak. Intake leak is usually nearer the center of the manifold.
I'm out of ideas.