6-71 vs. 8-71

So I'm trying to plan way ahead for my blower build (like one year ahead, lol) and was wondering if there is a way to pick the size of blower to put on the engine. I know generically that an 8-71 moves more air than a 6-71, but does it really matter which you pick? Can you just adjust boost via pulleys to get the same sort of air out of either (ie. drive the 6-71 harder than an 8-71 up to a point)? The engine will most likely be a 318, possibly stroked, but still deciding on that. There really isn't much else I can say about the engine now since I'm still planning it all out, aside from all forged internals, a main cap girdle, and probably a set of Edelbrock heads. It will be street driven, but raced on weekends. The current plan is to drive it around town with lower boost and swap pulleys at the track for big boost and use race gas.