The question really is what rpm do you plan on taking the engine too? If you plan on running over 7000rpm closer to 8000rpm then the 750 is what you want if not however then a 650 will do plenty.
That is kind of what I thought. I'm a little pissed I bought the wrong size. I have a speed Demon 750 right now and that is what the shop talked me into. The car runs pretty good but I think it would run better with the 650. The 750 only has a couple hundred miles on it if anyone is interested. Thanks for the help guys.