I don't have much experience in terms of larger tanks such as yours, but there's a very important concept that is being misused here. Watts per gallon is a useless way to measure light output when it comes to T5HO lighting due to the variation in reflector quality. WPG
is okay to estimate lighting if you were comparing T8, CFLs, etc.
I would PM Rivercats as she has expertise in lighting huge planted tanks. She has a 220g planted. But first you need to determine if you want low, moderate, or high light and if you're going to be using CO2
injection if you decide to go moderate or high.
If you want to go LED
, there's also buildmyLED.com. These guys are great and can also assist you in choosing the right lighting for your needs. They most likely have the length you need or can custom build it for you. I think Rivercats got a 72" fixture from them. It's personally go with spectrum geared to Dutch that BML has.