In my last set up, which was 36"x18", all I needed was 1 250watt MH fixture. It had VHO actinic supplementation.
You'd need two clamp-on lights for that size tank, as the spread isn't great, just incase you were thinking about that route. I have a Coralife 150w Clamp-On which is ok for my 30 cube, but would really push it for any larger tank.