Great info 65TerrorCuda and Bill Dedman. I stand corrected. Wondered what the cfm ratings really meant. Sounds squishy. Next question will be what conditions are 1-barrel Holley's tested under, especially since the OP has a Slant Six.
Finally, my Newport has a Holley Pro-jection "2-barrel" rated "670 cfm". Is is that comparable to a 670 cfm 4-barrel carb. It actually requires a 4-barrel manifold, just has 2 injectors in the throttle body.