+1 for ingg's suggestion. I think you could easily only use four 4 x 54 watt fixtures for a total of 16 x 54 watt 48" T5HO bulbs.You could easily do all T5's, and that is WAY too much light listed above, sorry, but I disagree on it not being too much.
I can grow almost anything in a 180g, 24" tall, 24" deep, 6' wide, with 8x39w T5's. With 12x39w, it is silly high light, I can't control the algae.
So you'd be using 54w instead of 39w, side by side 4' fixtures....
24*54w (2 6 bulb fixtures per "side", 4 total) is 1296 watts, and it'd grow anything in my opinion - and all the light probably isn't even needed unless you plan on a tank full of extremely light demanding species.