I think you might do better with a 950CFM.
If you plan on driving it on the street the smaller carb would be better. My 408 will only theoritically require a 850 @ 7000 rpm's. I'll be going with Brand New Holley Model#-9380 , its a chokeless 850(flows 900cfm) w/annular discharge boosters.
I'm running a 10.5 to 1 408CID w/TFS"R" heads, Lunati Steel Roller. Shooting for the 600HP range @ 6700-7000RPM.