5' is an awkward width for a ply tank as the sheets are usually 4'/1200mm wide.
Why would you want a tank that big 4' off the ground? Not only are you going to need a ladder every time you go to feed them (let alone trying to get in there for maintenance), it is also an awkward height for viewing unless you're planning on standing right in front of it. Seriously consider making the tank lower so you can easily get into it, and so it is a better height for viewing while chilling on a couch or armchair.
It seems to me like you're putting the house before the cart a little, how big is your tank now and how long are you planning to take building this monster tank? Obviously if you're starting with a juvi aro you can keep in in a smaller tank, but best intentions aside big tank builds almost always end up taking longer than you think. It sounds as though you have a lot of decisions to make regarding the tank size and construction method, maybe get a better idea of what you're going to do to house it then start shopping for the aro?