The correct SIZE is every bit as important as the brand. Carburetors are just "dumb" air / fuel mixers (not really).... and they depend on vacuum and air velocity etc. So if it's a STREET carb not an all out max effort race car... I'd use 85% volumetric efficiency for your calculation.
CFM = Cubic Inches x RPM x Volumetric Efficiency divided by 3456
396 X 6500 X .85 ÷3456...... 633 CFM
For a 396.... I'd run a 700 or a 750. You're always actually better off with a carb that is SMALLER than you are with a carb that is too big! Too big leads to poor response - poor fuel milage - and a sluggish motor.
Most of the factory "hi performance" 396's ran a Holley 780.... as did the 427's of the "day". My tri-power 427/400 hp ran triple two barrels that were 1150.
Remember that back in the day - fuel was cheap - and gas milage was HORRIBLE..... I prefer to think about how little time I spend driving a street car at wide open throttle.... versus just light throttle cruising.
You asked about brand..... Holley was the carb GM put on the hi performance BBC's in their hay day.
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