It is fair to say that oversizing a carb that uses an air valve AND small-er primaries is somewhat forgiving. The small pris will give good response, even if the throttle is mashed because secondary air flow through the throttle bores does not start until the engine is ready for it. If you use say a 850 Holley double trouble, with it's large pri bores, then the engine, unless it is a monster engine, is not going to need 850 cfm at 3000 rpm.