from everything I have read you want 2cfm per HP - - so Ramie is right in there - what I can't figure is Dave (Ddaddy) says I should be pushing over 500HP but for sure that 950 I had on there when I got the car was too much...
Well, I've always gone by displacement of the engine to establish carb CFM for NA applications.
A 408 CID is about 0.24 CFM per compression stroke. At 6,500 rpm you are pumping 780 CFM. Best carb would be a 750 CFM for street use or an 800 CFM for the track. More carb flow than the engine can use doesn't really help and may hurt.
After that, it's all about A/F ratio across the RPM range to keep you in the sweet spot.