Also the carb they stated only flowed 256 cfm which means they have to make 1.24 HP per cfm. There are things that can be done to the carb to increase airflow like thinned throttle shafts and plates and contouring the booster to name a few. Its not out of the question, not easy but will require a well thought out approach.
The engine isn't gonna only see 256 cfm, it would only see that if it was pulling 1.5"hg at full throttle but it will be pulling vacuum way higher than that, air is compressible, it's just gonna use more pumping power to do so.