how much head flow to justify dually carbs?

Until you get into high horsepower applications and things like tunnel rams you're likely better off with a single four. Dual fours were a necessity when all four barrels were small, now for most folks it's mainly for decoration. With that said, I do have a factory iron dual four intake for an RB (original 413 intake). I don't know if I'm trying to talk myself into or out of using it........