The issue isn't the nut but the mounting bolt on the brake. If it is short enough to end short of the far side of the fork by 3mm or more, with the thread ending about 10mm or so in, then all you need is the right nut. The same applies if the thread ends deep enough that you can cut it to fit.
But if the unthreaded section reaches to within 10mm from the far side you need a new brake or a new bolt.
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