Originally Posted by FBinNY

Dia compe made a special brake mounting adapter for these rear bridges. Someone posted a picture of one here on this forum about a month or two ago. maybe you can find a link to that thread and picture. Finding the actual part may be a bit more difficult, but there are probably thousands of them around on rusting frames of that era.

In the meantime you might be able to make do with 2 angle brackets, a 6mm screw, and a 6mm cap screw.