That is a tall tank! You will definitely need optics on your LED's if you do go ahead and setup one. I have the same 6ft tank as yours but mine is smaller in volume capacity, 135gal. My initial plan was to slap in about 48whites and 48blues on it, but ended up with just 36x36. I'd say that you can start with 48x48 across your tank, since your tank is wider than mine, and use optics for depth. You can always add/upgrade more later.
Good luck man!