BS.............. Mathematically speaking, a 421 cub with air flow increased 50% over the as-cast capability of the cylinder, will not reach mach 1 flow speed through 35mm carburetors, let alone 39mm carbs. If you were effectively able to come up with a way to move the air volume requirted with that setup, then the small bore pipes you are using will become a limitation.
If you make more power with that setup with 39mm carbs than with 35mm carbs, then your 35mm carbs/intake are setup incorrectly or you have ported the cylinders for peak air flow at such a high rpm that your torque output is not what it could be (there is no "free lunch," to quote John Judd when it some to horsepower vs. torque with a given engine displacement).
Maybe you have a problem with your dyno or dyno techniques? :ermm: