I use these units.
30" is indeed wide, I'd still run a single row down the middle.
An AI unit is 1' long. If you raise it a reasonable amount over the water surface you'll still get excellent par 4-6" from either end of the fixture. You're talking 20-24" of light coverage strong enough to easily grow high light demand corals like SPS.
If you're like me, and don't want much light on the glass surfaces I consider 30" (maybe 28") an almost optimal width for running a single row of AI fixtures. In fact my next tank, which I've already started ordering equipment for, is 72x30x26 and I'll be running a single row of AI SOL Blues as well. So you know I'm not blowing smoke.
Keeping my theme of not liking high light on my glass I'll run only 5 fixtures on my 6' tank. I probably would run 5 on yours as well, yes five, but you could go 6 or even 7.
If you absolutely must have nothing but high light zone over every inch of your tank then that's when you could start thinking 8+.