I don't know what the geometry of your Gitane is; the head tube angle, at least, looks pretty steep, and one of my 650B conversions, a 1978 Raleigh Competition GS, is probably similar. It went from a somewhat twitchy sports bike to a much more sure-handling sports-tourer. The only thing to make sure of is that your BB is high enough. What's the measurement from the center of the BB to the ground currently with 700c wheels?

Neal