To give a somewhat more technical answer:

When Carter was still building the AFB carburetor, they built 505 different for different applications.

For basically stock Chevrolet engines, Carter recommended the following:

265 (warmed over) - 400 CFM (9400s/9410s)
283 (stock) - 400 CFM (9400s/9410s)
283 (high performance street or mild race) 500 CFM (4758s/4761s)
327 (stock) 500 CFM (9500s/9510s)
327 (high performance street or mild race) 625 CFM (4759s)
350 (stock) 600/625 CFM (I forget the number for the 600/9625s)
350 (very high performance street or mild race) 750 CFM (4760s/4762s)

All of the above are "square bore" carburetors. None will fit the factory 265 or pre-1961 283 intake manifolds without adapters. The 9400s/9410 will, I believe, fit the 1961-up 283 factory intake; no others will.

Carter had other recommendations if the manifold were switched to a "spread-bore" (ie Q-Jet style) manifold.

Installing too large a carburetor on any engine will kill low RPM air velocity, thus requiring an overrich mixture. If the engine has been modified with compression, cam, ignition; the larger carburetor will permit a higher RPM, thus more power at the higher RPM.

This article discusses several of the various CFM rating systems:

http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

CFM requirements for a multi-cylinder four-stroke engine of at least 4 cylinders at a volumetric efficiency of 100 percent may be calculated at any RPM by the following:

CFM = (RPM times CID) / 3456.

I thought I had the explanation for this on our website, but it currently isn't. It will be!

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
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