Technical Carburetor CFM

I found a neat little, traditional air cleaner that I'd like to use on a mild build 327 SBC. The air cleaner has a mounting diameter of 4-1/8", while the carb I have is 5-1/8". Checking the area of the air cleaner, I calculated the square area is approximately 35% less than the 5-1/8" carb bore on newer carbs.
The carb I want to use is a 600cfm, so a reduction of 35%, based on the necking down of the adaptor I'd need and the area lost in doing so, would make it a 390cfm effectively - is that correct or am I figuring it wrong or leaving out part of the equation ??

If I'm correct, I'd need a larger cfm carb (750-850cfm), wouldn't I ? Would it make a whole lot of difference ? Am I over-thinking it ?

 

CFM is not determined by the air cleaner hole size, because that is usually not nearly the most restrictive part of a carb.

You just need a traditional old carb with a small air cleaner base, like a Carter AFB from the 1960s.

 

Rework the base so it fits

 

Your math is off.
Make a new base, or cut and weld the old one.

600 cfm is plenty.