650 cfm
Just jet it for the needs of the engine.
This way if you do some upgrades to the engine, the carb has the ability to be jetted to supply fuel for the additional airflow.
With a stock 289, you will need to be conservative, so that 500 CFM carburetor will probably run very well on the street.
The original 4-barrel Ford put on these was only about 480 CFM since that camshaft will not turn high RPMs.