If you can source the correct bolt in a longer version, then it's an easy swap. If you ordered a pair of brakes there may be an alternate fix. The front brake's recessed bolt is usually long enough for service as a nutted type in the rear. That leaves the very short bolt for the fork. Drill the back of the fork open to accept a recessed style nut (no need to counter bore for a flush fit). Then buy a recessed nut long enough to reach. (measure and buy the nut before you make any modifications).
BTW- if you go this route, don't forget to reverse the brake shoes back to front on each brake.
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