i use 2 x 400W MH on my 6' 180. and the corals in the dim area are doing just fine. but having said that, there is going to be a visible dim area, unless you are able to raise your MH a foot or so ablove the tank.

i am thinking of changing my lights to 3 x 250W MH, to provide an evenly lit tank.

having said that, i would probably go 3-4 250W MH and some 6' VHO.

with the 250s you will be able to keep a vast variety of animals.

PS. you'll love the 3' depth of your proposed tank. it will allow for some really nice aquascaping!