Considering switching to t5

[i have crabs]

since you have 2 250w halides now i would try to get 8 bulbs over your tank, i have a 48" tek fixture with 8 bulbs over a 28" deep tank and its workin out fine but never had 250's so i cant compare,problem is i dont think their is a 8 bulb unit for 36" also t5's still put out quite a bit of heat so they should be up off the tank a good 8-10" i would say, i know on the tek fixtures they have 2 switches so half the bulbs are on each which allows for staggering of the lights.
if you have to go with a 6 bulb fixture i would think you would be going down in lighting but its just a guess so you might nee 2 4 bulb lights and im shure that is not gonna look as nice and sleek as you where hopin for