They're not exactly the same, but you can make them work. Have you tried the Carter in your picture? It's only a few minutes to see if the symptoms are the same with another carburetor. I think it's been mentioned on this thread, but Edelbrocks are running on the idle circuit at a higher cruise speed than you might think. I tune these with the help of a wideband (just to see the neighborhood I'm in), and in my experience, they're lean just at the top of "idle" cruise, which may be up to 60 mph with a tall enough gear. Past that, they're running on the mains, and then the rods/jets come into play. My experience is also that they're rich under full throttle, but that's better than the other way around. I generally will reduce the size of the primary idle air bleed on top of the primary venturi cluster by .003, and that often takes care of any minor surging. But if you have backfiring where there wasn't any before, something must have changed. As I said earlier, I'd be popping one of the other carbs on there to rule out any problems with the 1405.