Agreed. You don't have enough light for most of the clams. H. Hippopus would be ok. I would even be a little hesitant at trying the lower light clams like derasa and squamosa as they like the sand and will most often jump from rocks. Maxima and croceas would definately not have enough light, not even high in the rocks.
As said above, 2 150w HQI metal halides would be the minumum for all clams in a tank your size....250s would be much, much more preferable.