Edelbrock Carburetor tuning question...

I have an older Edelbrock 625 CFM AFB style carburetor. It still has the Carter Competition series number it is such an early production carburetor. Anyway. it is on my 75 Ford F250. 351M engine, stone stock. I just changed out the plugs. All of them were white like it is running lean. What would be a good step up as far as a metering rod change? The carburetor is still stock and unmodified. Thank you drive through.