An ATI 36x6 bulb would do fine.
I have the 8 bulb TEK fixture with 4 ATI Blue Plus, 2 Aqua Blue Special, and 2 Actinic White.
I have never heard of a 36" T-5 fixure on a 48" tank. I have my fixture on tank mounts and it sits about 4" over the water. I would definitely have dead spots if there was no light over the last 6" on each end.
You can do that with MH, but I would not think it would work for T5.
I went with 7 48" bulbs with Ice Cap ballasts retrofitted in a canopy. I would do 6 48" bulbs.