Hi Russell, sorry I wasn’t paying attention to your earlier questions on carb CFM and I’ll respond because it is an interesting area that I've mucked around with a bit.
I reckon it’s useful to ponder a bit on how carbs are rated. At rated airflow capacity a 4bbl carb will lose about 5% of mass airflow across it, and a 2bbl about 10%. That’s oxygen the engine is capable of turning into power that is not delivered to it because of pressure drop across the carb. This means that an engine that is capable of pulling 750cfm through a 750 4bbl would theoretically pull 790cfm through the mythical zero loss intake system, and an engine pulling 350cfm through a 350 2bbl would pull 390cfm through the mythical zero loss set-up.
The “rule of thumb” carb sizing calculators are all based on assumptions about what is an acceptable compromise in mass airflow loss at the top end in order to be confident that the carb will do a decent job of metering and distributing fuel at low airflows and on transitions. As has been pointed out, a really healthy 215ci mill getting its ring rev’d out might pull 400 cfm depending on its VE. It’s not going to see about 40 of those pulling through a 350 cfm 2bbl, and will miss about 20 of them pulling through a 390 cfm 4bbl, and might miss out on 5 or 10 pulling through a 750 cfm 4bbl.
It’s obviously no good thinking theorectically about this if the larger carb can’t perform properly at lower airflows, however in my experience there is a substantial difference between the ability of similar rated Holleys and Holley-clones in this area, and it’s usually related to the quality of design and manufacture of the carb main body and the boosters. My rule of thumb for a mill that must be flexible and responsive at normal engine speeds is to use the biggest carb I can get away with that will deliver decent idle, low speed and transition performance. That’s because I don’t like the idea of leaving up to 10% of airflow on the table at the top end. Manifold choice is critical as is tuning but in most cases you can probably avoid losing any more than a few %. You’ll generally end up with a carb that might, on the face of it, appear a bit big for the engine.
Having said all that, something decent in the 390-500 cfm range is a safe bet and would work just fine!
Cheers, Andrew