Here it is,
395cu/2 =197.5
6000rpm/1728=3.47
197.5 x 3.47 =685 cfm assuming 100% efficiency,but take it you did a good job on cams head exhaust etc we will use 80%
685 x 0.8 =548 cfm
So if your carb is jetted and set right a 600 cfm will be perfect.Her's a link for you to find your carb and it will give all the correct jet and power valve sizes.
Good luck.It does not help to overcarb your motor!
http://www.mortec.com/carbs.htm