Carburetor cfm for a mild 318 build

The OP has not given the cam specs, only that it is mild. That means the volumetric efficiency of the engine will be low-ish.....
The VE is highest at the peak TQ rpm, which will be well below the theoretical max 5500 rpm of the intake. VE at 5500 might only be 80%, compared to 85% at peak tq. That equates to about 520 cfm for 318 cubes @ 5500. Better to err on the small side.