All clams will need a certain amount of Lumens to live and grow (provided if they also are in the correct conditions). The Lumens/intensity do not increase as the tank size decreases. 39wts is only 39wts, weither it's on a 6 gallon or 600 gallon tank. While it may appear better on a 6 gallon, the lumens/intensity are still the same on the 600 gallon.
I agree with Justin, If you want to keep clams, get halides. Plus, most clams will outgrow a 6 gallon anyway.
Take Care,
Graham