2 250W double ended mini pendents would be ideal for a 75 gallon tank IMO. Since you have an open top tank this is the easiest solution; jjust hang them from the ceiling and plug 'em in. They're pretty focused so you won't have light blasting all over the room, and the light quality is terrific. If you want to build a canopy, you can do the retro fit as suggested, but make sure you get quality reflectors, and it does have the advantage of being able to use VHO actinics to improve the color. But, there's a good heat advantage to an open top tank in this climate, and there are many excellent bulbs to choose from so the actinic support is not as important as it used to be.

Most people would disagree with thunderkat on this issue; for a reef tank, especially clams, MH lighting is the preferred choice. The shimmer lines come from the light being a point source, and are considered very natural and desirable; like sunlight making patterns on the ocean with the waves.