Couple of other thoughts from the King of letting things sit a long time and then resurecting (me). Cracked dizzy cap, especially since you changed out that component, you say, I'd really give it a look over. Mouse nest in exhaust and mud dobber nest in intake have been some issues I have seen. If air cleaner on whole time, hard to imagine dobber nest (will cause lean running)and mouse nest probably would not cause backfire. I had some valves get rust (I suspect rusty on seat) and caused some backfire, just drive it on a long drive 20 miles or so. Sticking valves, you mention can cause your problem. Hook a vacuum gage up with a long hose into driver compartment as you drive and monitor when it acts up, needle will bounce around(could be another valvetrain issue too like spring). All these steps are non-invasive in keeping with the KISS theme. Then, dig into carb when all these check out.