I bet that Gitane had Campagnolo lever bosses. They are notorious for causing that down tube failure - they create a stress raiser at the corners.
Cantilever brakes can be very powerful but they need a good lever as well as a good setup. Unfortunately I've never been able to get really good cantilever braking with any road levers, although they can be made acceptable with decent pads. The old DX 4-finger levers for flat bars, on the other hand, give you brakes that work almost as well as discs.