From a purely theoretical standpoint, a scientifically designed two-barrel intake COULD provide a measurable (at least on the dyno, probably NOT by the seat-of-the-pants machine) increase in engine efficiency assuming the two-barrel used is something like a Carter WCD with metering rod technology, and the metering rods are custom cut for the application when comparing to a single barrel.
From a practical standpoint: WHY???? Single barrel carburetors exist in a sufficiently large size to feed ANY street 216/235/250/261/292 Chevrolet engine; and if the single barrel is properly selected and correctly tuned, the two-barrel advantage is going to be limited to a very few percent.
The two into one adapter annoys the carburetor, as it defeats the original engineering design, at least for a conventional 2-barrel. Air velocity in the venturii creates a negative pressure in the fuel delivery points allowing atmospheric pressure to push fuel into the negative pressure areas and thence into the venturii area. The air velocity is normally controlled by the throttle plate opening. The two into one adapter upsets this design, and as Jerry mentioned, has been proved many times to LOSE performance when compared to a decent-condition and properly tuned 1-barrel.
However, from a "bench-racing" or "parking-lot-fast" standpoint, it can be any lie you think you can get away with
And, again as Jerry mentioned, due to the port design of the cylinder head, the single two might be better than 2 singles. But if you REALLY want to increase the low RPM volumetric efficiency of the Chevy 6, then design a 3 carb manifold where each carb feeds 2 cylinders; BUT WAIT - didn't Chevrolet do that with the Corvette?
As a good friend on another forum is so fond of posting: "engineers do things for a reason"!
Jon.
