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.
My new brakes came in last night. Looking great, and I had no trouble installing the front one. As a matter of fact the old Dia Compe did have a recessed bolt already!
However, the rear did not, and Sheldon Brown's site only says this:
"It is most common to install a rear caliper brake behind the brake bridge, but locating it ahdad of the brake bridge has advantages. sometimes a brake behidn the brake bridge will interfere with a rear rack.
Recessed mounting, nuts and bolts. Drilling a frame." But it doesn't fit in the hole ahead of the bridge either, and would have to be drilled out. I thought he knew a way without drilling?
Never mind, I put in a morning of intense afro-engineering, and managed to drill out the hole. Without proper tools, skills, or experience I had to fall back onto one of my few strength - perseverance. It all came out in the wash, looking good, and working!