72" T5 high output lamps are not available anywhere. A ballast doesn't exist for it.
60" is the longest you'll find, and that's actually what I would use on your tank. Fluorescent lamps spread light well enough to where you wouldn't notice the 12" in your aquarium without the bulb directly over it. I'm running a 3' 6 lamp on my 4' 90. You would never be able to tell if I had the canopy closed.
60" lamps are the brightest lamps inch x inch available as well. They run on 80W each. If you doubled the lamps, say you decided to run 3' lamps side x side, that's still less wattage, 78W. Then you need to start considering lamp replacement costs, and it would just be expensive!
I would run 4 x 80W as supplemental lighting. Giesemann is the only lamp manufacturer making these bulbs, so I would pick 2 80W Pure Actinic, and 2 80W Actinic+ One of each of either side of the halides, preferably Actinic+ in position 1 and 3 front to back.
You can run a pair of Ice-Cap 660's or Workhorse 7 ballasts for this. Advance makes a 1x80W ballasts as well. The Workhorse 7 can run 2 80W T5's, and is the cheapest option. (Works great though)
Hope to help.
Sam