here's my 2 cents:
393 c.i. is .2274 c.f. and as a 4 stroke you are only intaking half that volume per revolution, thus .1137 c.f. and at 6500 rpm you are looking at 739 CFM, so i would check into a 750 CFM.
Good Luck,
Walz
EDIT:
Just remember that carbs are depend on velocity to...