also, why is your overflow box so wide? Is there a specific reason for such a wide overflow box? There seems like a lot of space in there to collect detritus. If you plan to cover your overflow with grates I would imagine that much overflow is going to be noisy. Imagine dozens of tiny waterfalls splashing into your overflow box. If you don't plan to cover it, imagine fishing out fish from your overflow box often
Anyway, it seems to me like you could easily get away with an overflow 1/3 that size.