Use a long bolt and spacers. Blackburn racks used to come with aluminum spacers for such purposes.

Doing so will put some leverage on the braze on, and the bolt, if not completely tight, will wiggle in the braze on, loosening more and doing damage. Whenever i need to space out a rack, I install a long bolt from the inside out, so it is screwed tightly against the braze on. For extra piece of mind, fasten a nut from the other side. Then you have a length of bolt really securely attached. Use spacers or washers, then the rack, then a washer and a nut.