I'd recommend measuring the diameter of the top tube before you get a whole lot of time and money tied up in this. Older French bikes used a 26mm diameter top tube, vs 25.4 (1") for everybody else in the world. Gitane might have switched to 25.4 by the early 80's, but you'd do well to check it, as metric frame tubing is pretty much unobtanium.

SP
Bend, OR