72 Duster, Ethanol gas, intake backfire

Here's hpow I look at it.
It takes three things to get combustion.
Vaporized Fuel, O2, and something to ignite it.
So really the only way for backfire through the carb is when there's spark before a intake valve shuts.
So yea -I'd first check the ignition system for something that will cause early spark. And then sad to say - the valve train.

All our current pump gas has poor storage and looses its light end easily. The light end of fuel is the stuff that vaporizes easiest. Think of it like the tinder and smallest kindling when building a camp fire. Take that away and its much harder to get a good fire started. My experience with old fuel is that the engine misses, and with winter fuel in the tank when the engine is hot sometimes it will just die -especially under load. Doesn't sound like you've got either of these. The other issue with current fuel you've already looked at - sometimes its components attack rubber check valves, diaphrams etc.