Strictly in the realm of idle speculation:
Carter did NOT publish a CFM rating on the 2066s type YH carburetor(s) used on the 6 cylinder Corvette. Chevrolet was happy with them, and it wasn't anyone's else's business
However, the main venturi was 1 5/16 inches.
In Doug Roe's book, a monojet with 1 5/16 inch venturi is listed as 210 CFM.
1 5/16 translates to about 33.3 mm. Zenith shows a size 3 carburetor with 31 mm venturi as flowing 210 CFM, and a size 4 carburetor with a 34 mm venturi as flowing 220 CFM.
All of the above would be at 3 inches of Hg. Converting to the 1 1/2 inch Hg (4 barrel scale):
210/1.414 = 148 CFM
220/1.414 - 156 CFM
CFM in a multiple carburetor setup is not "additive"; you have to throw in the IFF (a.k.a. International Fudge Factor ;)) which varies for different manifold designs.
As stated above, idle speculation.
As to the 4-barrel, I put 400,000 miles on a different make (rhymes with Cord) 300 CID 6 using an Offenhauser intake and 400 CFM Carter AFB (number 9400s). The engine exhibited good drivability, and the fuel economy normally associated with Carters.
Jon.
