Personally, I don't think it is POSSIBLE to create light that is too bright for tridacna clams (at least not with commonly available reef bulbs, even at 400w). I have seen clams on the reef flat in Polynesia that were exposed at high tide they were so shallow. We're talking direct tropic sunlight at noon in about 4" of water (the water being over 85 degrees, as well). The only thing I would be worried about with clams and bright lighting is reintroducing them to bright light after an extended period of darkness (like immediately after they have spent several weeks in wholesale / shipping).
Clams in the wild are brilliantly colored - very very dark. They have algae deep within their body tissues. If your clam is pale at all, anywhere, it could probably use brighter light.