Solved the stumble / rough running on my '68 OTS

Gentleman...I'm happy to report a tentative resolution to my woes. The short answer is that my exhaust valve timing was off by 15-20 degrees due to the camshaft "adjustment sprocket" exhibiting stripped teeth and the exhaust cam subsequently rotating out of adjustment. Just for interest, here were the trouble shooting steps:
1. Carbs rebuilt by me and second checked by a pro carb shop...AOK.
2. Replaced all ignition electrical items, coil, cap, plugs, rotor, points, wires...AOK
3. Double-checked firing order by "running" the ignition wires from plug to distributor (a side note: my wires run through a cardboard manifold or sleeve, and I discovered that you cannot check this via a multi-meter as there appears to be a diode? or small fuse looking item in the sparkplug boot that prevents a continuity check).
4. Ignition timing checked and double checked at 10 degrees BTC at 1,000 rpm.
5. Dis-assembled, cleaned, and oiled the distributor with it's centrifugal advance mechanism. Other than a little "stickiness"...AOK.
6. A visual check of the plugs revealed 2,3,4,5 looking light tan and clean whilst 1 and 6 were black and sooty...obviously poor burning of the fuel/air mixture. As the electrics were previously checked okay and carb function would affect all cylinders, I decided to check the valve timing.
7. Timing chain tension was checked...AOK.
8. The last thing to check: with #6 cyl at TDC, the inlet cam was checked AOK with the Jag cam tool...the exhaust cam was not even close. Huh? While trying to adjust the exhaust cam sprocket, it was found to rotate rather freely when torque was applied to the nuts with a wrench. Further investigation revealed nicely rounded teeth on both the adjuster ring and the adjusting plate.

Parts are on order and I'm confident that things will be back to normal shortly. Thanks again to all who offered advice...it definitely helped throughout all the troubleshooting.

Cheers, Eric.