6-71 vs. 8-71

Lot of good simple information on the Blower Drive Service web site.
http://www.blowerdriveservice.com/#

I think the simple answer to your question is that the 8-71 is to big for your motor size. It will require more HP to run that blower on your engine netting you less HP overall.

If I remember right the 6-71 is good for up to almost 430 cubic inches in most cases.