On my own 125, I got a Catalina 48" 4x54w fixture because it is such a great price. It works really well, but if I had the money for it, a 72" 4x80w fixture would be my choice now. But 2 36" 4x39w would work, but not as pretty. I haven't looked at prices everywhere, but at Catalina, the 72" is cheaper than 2 36" fixtures.
I think the best way to light that tank is to get a 4 bulb 72 inch fixture, too, but I would definitely hang the light above the tank so I could raise it to lower the light intensity. Raising it 6-10 inches would probably be about right. The 18 inch front to back depth makes it hard to get uniform light on the substrate without multiple fixtures separated by a few inches, or by raising the fixture so the light spreads out more.