Steve I'd begin with 3x 250w. Keep in mind I have 2x 250w over a 75g which is 48x 18x18 (I think if I remember right this early). The halide rule of thumb I learned was that if measuring from the center of the bulb expect about 9" of good spread each direction, so an 18" diameter. Place the first bulb 9" on center from the edge of the tank and then 18" or less apart from there across the entire length.

I come up with 4 lamps being perfect. Now if I'm thinking correctly this early in the morning, Place the first lamp at 9"OC, 27"OC, 45"OC, and 63"OC. This would allow for 9" off one end, then 18"x18"x18" with 9" to spare on the other end. This all works out well for a 72" long tank. I'd maybe even think about getting screw in bulbs as the inensity is spread longer than a more direct firing dual end bulb. For that tank you'd want long parabolic style reflectors and the expensive Lumenarcs don't seem as though they'd help much since they are made to project over a large square.

The 18" rule was handed to me to prevent dark spots between the bulbs. To save money why not do as I plan to do with my 400g tank? Since I cannot afford to run 4x 1,000w lights to gain the same 10wpg I currently have, instead I plan to have 1 very bright area that tapers off to a more shaded area. This plan will allow me to have SPS in one corner of the tank and have plenty of areas for LPS and shrooms that prefer lower lighting. You could group up 2 or 3 250w retros in one area spaced 18" apart but have them farther from the edge of the tank and let the PC's warm those areas up with a little less intense light. Those areas will still be bright enough to look at and enjoy the LPS you place there but not as intense as the halide areas.