The engine, falls flat on it's face at at 5400 and doesn't have the power of a street 302 @ 5500, why are you talking about 6000 rpm shift point?
Ford used the 780 cfm carb, not for the street version of the Boss 302 but, so that it would be legal for the 8000+ rpm Trans Am engine.
"Street engines are way different than all out race engines but, even race engines, need the right A/F, in the range they run in, even if it's only a 1500 rpm band. (Edited to read band, I wasn't referring to a race engine running at 1500)
For a race engine, you give it all the carburation you can, until the HP drops off, in the range your working in. In the engines that I had dynoed, they jet for max HP and then, come back with the air correcting jets, to bring the mid range in."
As you can see in this part of my reply, I'm agreeing with you guys, regarding a all out engine.
For the guys that reference the large carbs, are the engine build as streetable 5200 rpm engines and what was the power like at 2500?
What I'm suggesting is that the 1500 - 4000 rpm number, just might pick, w/o loosing the 5200 max HP. It stumbles, right out of the gate, on the print out.
As for my own experience with a race engine, the car in my avator is a twin cam Cosworth, it produces 200 hp, out of 96 CI, at 8500 rpm and has a pair of 600 cfm Webers so, I do know what large carbs can do for a race engine but, idles over 1600 rpm and stalls if given any load under 3500. If I where running it at 5200, I'd probably run 150 cfm, not 1200.
I'm only suggesting that it could be tried, as street engine and we won't know if it would produce better lower end numbers, w/o that.
It looks to me like the cam is what restricts it's air flow not the carb.