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I'm not totally sure that @AlexD's suggestion is on the money. It depends how long the bolt on the old rear brake is, but it's not ideal to have the sleeve nut and pivot bolt meeting so far forward. SJS sell the original pivot bolts and from the look of them you could do a proper conversion with the JTek bolt and stick with the nut-fitting. Spa also do a version of the same thing in stainless.
Do you have both brakes, and you're fitting them to a non-recessed frame by any chance?
If so, what I've found is the simplest way is to swap the brakes round and use the front brake on the rear so the bolt is long enough. Then drill the rear hole on the fork and use a longer (recessed style) nut that can reach the threaded part of the shorter rear bolt.