In round numbers, 269" at 90% VE turning 4,500 RPM needs about 347 CFM, if rated at 1.5" Hg (like 4 bbl.).
Using 1 or 2 bbls. about 491 total at 3.0" Hg, for 2 carbs on a common manifold divide by 2.
The .7 correction is the square root of the difference between the 2 rating pressure drops.
The power figures are very interesting, but they can't take into account relative manifold efficiency (how restrictive, plenum volume) or mixture distribution imbalance (3-4 port is richer than 1-2 or 5-6) - those are pretty much decided by the manifold manufacturer unless you want to do some surgery.
