Found this on their site thanks Jerry:

There are two different 671 blowers commonly used. They are the larger diameter blower and the small diameter blower. The large bore blower (case thickness 1/2"-) is approximately 11 percent larger in volume than the small bore. The small bore blower (case thickness 3/4"+) is recommended for 370 CID engines or less for street applications. The larger bore blower is recommended for 371-500 CID engines in high performance, racing applications.