Mike,
My first question is, will both carbs. fit on the same intake? I'm thinking not, but I'm not totally sure here.
I think we might be dealing with spread-bore vs. square-bore here. I hope I'm wrong. In any event, the double-pumper is amazing. I think on the strip, that is the way to go. 650 wouldn't be too big for your engine, and you can drive it on the street. Seriously, as long as you don't get on it, you are only working on the primaries, and gas mileage won't suffer.
Mike