Modern calipers on old frame
I'm building up an old steel frame. I've got most of it working ok now, but the calipers I have refuse to fit. The back part of the bolt will not fit in either the hole on the forks oron the frame. Am I able to buy the back part in a slightly smaller size so I can get these brakes working, or do I need retro specific brakes? If so,some recommendations would be great!