Solving backfire problem on passenger side

[HSSS427]

Have a 427 side oiler, with Shelby aluminum heads and a Mighty Demon 750. I continue to get a back-fire on deceleration, and sometimes in between shifts (again a decel as I let off the gas). It only happens on the passenger side. I've tried to tighten down the header bolts at the head to make sure I'm not getting an air leak there. Using copper gaskets. The headers are individual pipes that connect into the collectors just before the pipes exit the body. The collector pipes slide into the header pipes and there are 2 bolts to hold it together. Was wondering if this could be the source of air, and if so, what can I use to seal this connection that will endure the heat and won't blow out.

Also, there's a flange that connects the collector to the side pipe with the idea that you could easily unbolt the side pipes to run straight headers. The flange connection is just about under the front edge of the doors. Is this too far down the pipe to cause a back-fire issue?

If it's a carb adjustment issue, what type adjustment should I be looking at, particularly given that it's only happening on one side of the engine. Float height? Jets - larger or smaller?

Finally, is it reasonable to expect this engine to run without back-fires or is that just nature of the beast?

Thanks!