The flow ratings for the rochesters was 600 CFM for the small blocks and 800 CRM for thje big blocks. Also the rochester is a spread bore type of carburetor whereas the Holley is a square bore type. So if you want to run a Holley you will either have to change to a square bore manifold (recommended) or use an adapter plate. Adapter plates restrict air flow and also raise the carburetor and can cause hood clearance problems. Also if you are using the stock air claner you will have to replace or modify it to fit a holley's top air horn