72" is just a nasty number for T5 lighted tanks.
I don't know if anyone makes a 72" T5 bulb. I've only seen them up to 60".
Personally if I had to light a 72" tank, I'd use a 60" light fixture.
Sure, there will be a slightly dim spot on the sides, but you can easily put lower-light corals in those spots. Caulastrea, mushrooms, leathers, ricordea, ect.....
I light a 20g (30") with a 24" fixture. There is a 3" gap on each end, but with the light on, you CANNOT tell the light is shorter.
I would be willing to bet that the 6" gap that a 60" fixture over a 72" tank would be fairly negligible, especially toward the bottom.
True parabolic reflectors would be a requirement for sure though.
As a side note, if you want more light distribution on a 72" tank, I'd personally be tempted to use a 3X250 MH light with two 60" T5's.