The 8:71 is just like a 6:71 except aprox 3/4 inch longer---hardly any difference unless you are running it at 45-50 % over---
a large bore must mean that they have over bored a 6:71 case and come up with some sort of roter to fit it---however there is a standard distance between the rotors so the gears mesh properly so they must have a completely different blower and are just calling it an big bore 6:71--this would take different endplates, gears, cases, rotors---everything but maybe they use the 6:71 manifold bolt pattern---i would like to hear more about what they have done