A 600 should be fine for a mild/moderate 318, especially on the street. Now, if you're racing it could be a different story, you'll want every last bit of that top end horsepower. On a built 318, a 650, or even a 750, could still improve the top end numbers. Using the above equation, a 340 doesn't need much more than a 600cfm carb. And yet, Mopar Muscle did an article where they gained 35 hp by bolting an 800cfm carb to a stock 340.
But keep in mind that horsepower isn't at the bottom of the curve, its at the top. You won't see it until higher in the RPM range (in the case of the article, things really took off after about 4,600 rpm). At 3k, the bigger carb gained 10 hp. At 5600, it gained 35 hp. But, with a street car, where are you spending your time? Most of the time, you'll probably be below 4,500 rpm, which means you won't see the majority of that increase in everyday driving.
And, if its mostly a street driver, you'll appreciate having a 600 instead of a 750 when it comes time to fill up the tank.
