A lean mixture can cause backfiring because the lean mixture burns more slowly, and may still be burning in the compression chamber when the intake valve begins to open.

Carburator mixture is not really dependant upon fuel pressure, as the carb requires a steady flow of fuel (not pressure) into the atmospherically vented float chamber (it is not pressurized), it is possible that low fuel flow could result in a lean mixture and backfire. I actually rather doubt that the pump would be the cuplrit of a overly lean mixture, its a long shot. Much more likely is an intake leak, but you have already checked for that. I suppose the intake leak could be internal to the engine, but I have never encountered that.

I am surprised that you don't have an engine driven mechanical fuel pump. The mechanical pump will likely out-flow any electric pump, and the big block needs its gas. Electric pumps are typical of fuel injected engines.

I also worry a little bit about a bad cam. A cam lobe going "flat" or changing shape could do this as well ( I have experienced this).