Since the bolt is a special one of fixed length you will need a longer nut. The recessed nut which threads onto your mounting bolt can be had in numerous lengths; you should be able to find a longer one which will reach between the bolt and the rear of the fork. A local shop should be able to provide them or they are available online. Follow this link for a photo of some available nuts: Aluminum Recessed Brake Nut 35mm | eBay