you might want to measure first or experiment with an old caliper brake first, if you haven't already. i think on all my conversions (one american made frame from 79 and one taiwanese frame made in 85) i managed to get away with what used to be called a standard, now long, reach brake of about 57mm. and aren't there some braze-on hangers that will be unsightly if not removed? i may very well be wrong about the hangers.

good luck with your project.