Hey bro,
You have to 1st determine if the noise is coming from within the overflow pipe, or at its outlet into the sump. If it's the latter, then are you using a metal stand for your sump, or an enclosed cabinet? This is because a cabinet help to contain much of the noise within.
OK, let's talk about the overflow box. IMO, if your existing pipe fails, even a customized one doesn't guarantee noise will be within your wife's acceptable threshold. Wouldn't it be easier to sell your existing setup & get a one with built-in overflow. Then can you be sure it'll be silent running.