[Chaplin]

Assuming your max rpm is 6500 and 100% volumetric efficiency (VE) (which you will not achieve with a naturally aspirated motor), you would need a carburetor that flowed 767 cfm.

How To Calculate CFM:
Engine size (CID) x maximum RPM / 3456 = CFM
CFM @ 100% volumetric efficiency

(Your example: 408 CID x 6500 RPM = 2,652,000 / 3456 = 767.4 CFM)

Below is from Holley:
Most Street engines are capable of achieving only about 80% VE; a modified street engine with ported heads, headers, intake and carburetor can achieve about 85% VE; a fully modified race engine can achieve 95% or greater VE. The CFM number arrived at with this formula must be factored by this percentage.

Therefore, assuming you achieved the same VE as a fully modified race engine, (95% VE), the max CFM necessary would be about 728 (95% of 767). Therefore, the 750 would be fine. You could use the 850 also, but you may suffer some loss of power and responsiveness on the bottom end because of decreased velocity of the air/fuel mixture.

FWIW, I just went through this drill with the techs at Barry Grant and they suggested a Might Demon 750 for my 428 with the comp cams 292H. I just bolted on the card tonight, but didn't get to test it out yet.