It looks like you have checked each part three times or more. It is unfortunate that you do not have a spare carburetor to swap out and eliminate it at one swell foop.
The following are checks I thought of but discounted as unlikely or you probably already checked.
A. Failing coil no. You don't have Lucas coil which will do this.
B. Mixed up plug wires. No. It does not backfire and you triple checked it out by observing rotor direction and cylinder position by checking the valves to see if they were in exhaust or intake mode or closed.
C. Timing not too far advanced. No. You checked it and the vacuum advance by disconnecting it with the engine running and trying to speed the engine up. Timing light checked all this out.
D. Bad plugs? No. You did not put Champions in it. They sometimes come defective. Rare, but I have seen it.
E. Bad distributor cap. no. There is no carbon tracking and the rotor strip is not too far from the studs. The carbon button on cap shows signs that it is contacting the brass strip on the rotor.
F. Blocked exhaust pipe. No. You checked it to make sure it was not restricted or dead rat in the pipe. Engine does not sound like exhaust blocked.
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