There is an equation that you can use to estimate what size of carb to use based on max RPM and engine size. It is RPM*(Cubic Inches)/3456. If you assumed a max RPM of 6000 RPM for your setup, 6000*427/3456=741cc. This assumes 100% volumetric efficiency, which most street motors do not achieve, so in that case it is a little bit of an over estimate. According to that equation, I think a 750 would work well. If you wanted it to breathe a little more on the top end, you could go with a 800 or something like that.
Adam
