A double pumper carb is fine if it is sized right. The vac secondary carbs only work right if you have enough vac to operate them and have the engine tuned really well. I guess what I am saying is the double pumpers are a little more forgiving with regard to engine tune. Your math seems right to me for a 600 to 650 cfm carb. I personally would go with a standard performer intake and a 650 double pumper carb. The e-brock carbs are great but as far as I know there isn't any 650's in the carter/weber style. Of course all of this is dictated by your cam and heads. You said it's a mild cam, headers ...
Is there any port work on the heads? What's your driving style? (always in the gas?) Is it your daily driver/commuter? What kid of shape is your ignition system in? Getting full spark and is it timed right?
Look into a 650 DP after you make sure everything else is up to snuff and I think you'll be really happy with your cars performance.
Eric
