IRO Mark V, Karate Monkey half fat, Trek 620 IGH, Cannondale 26/24 MTB, Amp Research B3, and more.
older nutted brakes on newer fork
In a fit of desperation I had to convert a threadless fork to threaded. One unforseen consequence was that the newer style fork has the recessed nut holes to mount the brakes. I would really like to use an old nutted brake for that wheel.
The bolt for the old caliper is much smaller than the recessed hole on the fork. But I have already found a nice little sleeve that fits snugly around the bolt and snugly in the hole. It even has a shoulder on the sleeve to keep it in position one I tighten down a nut on it.
I cannot think of any reason why this will not work or be unsafe.
At www.rivbike.com in the online store under brakes they have a picture of plastic sleeves they sell for exactly your purpose. Go look at them. If your sleeve is the same you've already solved your problem.