Will repeat here what I wrote elsewhere...

"...and have gone through long and repeated setting and re-setting of the timing, valves, and carbs. Everything appears to be working properly..."

Did the timing mark jump around for six months while you were wandering around your engine's pieces/parts?
If so, was it wise to ignore the clear indication that your distributor and whatever that petronix ignitor thing is was not working as well as your visual inspection indicated to you?

If good uniform compression, it's a waste of time to touch carbs until ignition system is functioning correctly, and yours isn't...

Assuming everything else distributor wear-related has been eliminated eg obvious shaft wear/play, distrib gear, etc, your problem is most likely with the weights/springs controlling advance, that's why it idles fine (weights not in play), and that's why it overheats (jumping-around timing mark=erratic advance=overheating) and backfires.

But I don't have any way of factoring in the electronic add-on, which I've had no use for in 40 years of Amazons and Bosch disributors.

Timing lights and dwell meters are wonderful things, and they don't lie...
Good luck,

Michael