How much horsepower for 318?
The 570 Street Avenger is more than enough carb. I have a 600 cfm Holley on my 360 and I also have a 725 Demon and a 750 Holley. The two bigger carbs really only make a difference above 4000 rpm and give up throttle response below.
With your set up a bigger carb will only net you worse gas mileage and worse low rpm throttle response.