how much cfm in a carb should I be looking for?

408 stroker 2800 243 HP and 458 LBS Torque..4200 RPM 406 HP 510 Torque. 5700. 492 HP and 442 LBS Torque. And I'm running a Holley 670 street avenger per engine builder. There's times I think about a bigger carb. But except for some idle adjustment I think it does pretty good.