You need to talk to Greg at reefgeek, TX, he's the real expert. Give him a call, he's great about service and talking things through...
BUT, I do know that LED's have limited spread (their light tends to go straight down and does not travel out as well as with other point light sources), and one unit can just handle about 24", and that's it. What I think the best game plan for the Aqua Illumination units and a wide tank like yours is to get two 36" units, and then mount them going front to back, across the width of the tank. At 24" spread each, that would cover the 48" length of the tank just fine. Or if you wanted to be really covered, which I don't know is even necessary, you could get two 48" units and mount them traditionally lengthwise. They would be covering only 18" each, so you are assured of GREAT coverage (at rather higher price too). It really depends on your rock layout and how crowded, or modern and open, you plan on the tank being. I really do like the idea (and the lower cost) of having two 36" units and mounting them parallel over the width of the tank...
Talk to Greg (tell him Todd said Hi) and see what he thinks...