I have a 71 Firebird with a 400, Lunati cam(mild), Edelbrock performer intake #2156, and a Holley 80457 that is 600CFM. What size carburetor should I use and which is compatible with the intake? The existing one seems to be weak at take off and will bog down when I give it gas after sitting at a traffic light.  I tried an Edelbrock carb and the mechanic could not get the motor to run correctly. Thanks for any ideas!  

sounds like the carb. is not adjusted right could also be the timing is not set right as well how much lift does the cam have.  

The lift for the Lunati cam is .490. The RPM range is 1800-5800. Holley technical support recommends a 670 CFM and their website recommends a 750 CFM. Will I see a significant difference if I upgrade to either one? Thanks  

You will not see a differance going from a 600cfm to a 670cfm carb. but you will see a differance with the 750cfm and that is the one I recomend using.