Updating this thread. I just pulled apart my 1972 Schwinn Sports Tourer and found that modern brake cable housing is a little too large in diameter to fit into the stops brazed onto the frame. The difference is small. I bet I can sand off some of the outer plastic on the housing to get it to fit.

Modern shifter cable housing fits just fine though into the brazed on cable stops. Jagwire sells a shift cable that will work with the Huret shifters,

https://www.modernbike.com/product-2...RoCMaMQAvD_BwE