We have used the ATI steel 7" balancer on 632" engines 4.750" stroke and at times over 8,000 rpm with out any issues. This is the balancer that ATI recomended that we use. ATI makes balancers for BBC in 6", 7" and 8 inch. I'd would think that any of there 7 or 8 inch units would work for you.
8 inch has been standard knowledge since 1970,s built by GM engineers for high hp oem bbc,s and sbc ,s as well. Take it from the best of the best and then add ATI to the subject one step beyond. Proven fact 8" dampens harmonics on 396/375hp and up and dont look back.