Try getting a smaller, cheaper clam, before you go and spend $160 bucks on one. If it would unfortunately die (which I don't think it would, your lighting sounds pretty good) anyone would prefer $30 clam to a $160 clam. What corals do you currently have in the tank? And how well are they doin? 55k metal halides could be a problem, so you might want to get a mh light with a higher kelvin value just for the third of the tank where the clam will go.
Clams can live very long and become very big, but odds are that in an aquarium it won't, so don't worry too much. Check out this pic of a huge clam near the Solmon Islands in the South Pacific
Good luck and keep us updated- Mark
[ December 08, 2001: Message edited by: Mark-24 ]