You won't need 3-4" behind the tank... if you do a glass holes kit, you'll need 2-2.5".... and you'd want that anyways. While I agree with Dougs reasoning behind the overflow box on the back, I think a full length overflow box would help a lot with noise...if it bothers you.
FWIW, I'm considering buying a new 40b and putting a corner overflow in it to replace my current tank...
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