The rear brake bridge and fork crown needs to be drilled out to the diameter of the recessed nut. If such an alteration is unacceptable (ie. expensive, sentimental bike frame), Sheldon Brown's site explains his method of installing modern calipers to old frames. Basically, the front caliper is mounted to the rear, and the rear caliper becomes the front one, with the nut tighten "inside" the fork crown via the opening of the steerer tube below.
Hope you can visualize it.
Not yet, but it may become clearer when I see the parts.