Can 48" fixture light 72" tank

well if you hang the light high enough you might be able to reach both sides of the tank.. the only thing with that is the higher you hang the light the less efficient it is
Personally... i would get the bigger light.. but thats up to you on how much money you are willing to spend
Is all this light for a salt or freshwater tank..
Cuz thats a heck of alot of light for freshwater

could always hang the 48" and buy some CF for the ends, just some shortever 12 inchers or 22" on either side, slightly infront of the 48"

With 2 250 MH lights, you could hang it from the moon and still grow plants. Personally I would fire up a 90 galllon tank and put that light over it. Add some sand, live rock, just about any coral you could think of, and some salt water.

The fixture and the bulbs' color temperatures are probably suited for saltwater.
Get a 72" fixture with better color temperature, and a tad less light.