Coralife Lunar Aqualight Deluxe 48"

If you really want to keep clams, my suggestion would be to go metal halide. If the pre-fab fixtures are too costly which they are, you can purchase the bulbs, fixtures, ballasts and cooling fans to make up your own system at a fraction of the cost. This is what I plan to do in a couple months. However, including a chiller kinda goes along with the metal halides right?
But stick to your plan of keeping the clam close to the top. Again If I'm wrong please correct me. This is a learning experience for me too

 

http://http://www.reefspecialty.com/Lighting/c14/index.html

Just found this link right here on 3Reef maybe this would help you to get an idea of the cost of fixing up you own lighting system. My advice would be to go with http://www.reefspecialty.com/Lighting-Complete-Retro-Kits-and-Pendants/c14_29/p200/CoralVue-400W-Metal-Halide-Complete-Retro-Kit/product_info.html and still continue to use ur regular lights and keep the halide directly above ur clam/clams. HTH

Cheers