Personally, if I were you I would just make a sump. You're going through all the trouble of an external overflow anyway. Will save you all that space behind the tank. I think the tank will look funny projecting out that much from the wall. Plus I think it will look messy being able to see the fuge and overflow through the tank. Also, not sure what you're planning for lighting, but I would make it either 24" or 36" long so you could use T5s.
Just my thoughts, but whatever works for you, right?
