Chas certainly knows his stuff especially when pertains to Gitanes and other FR makes. I would follow his very well-presented advice and also do your own "GO, NO-GO" test fitting as you intend, and watch the gap at the back of the seatlug for no pinching.
I am really happy with my own US model 1974-ish Grand Record (full 531DB and the black/red-orange paint scheme) it really delivers on the "French Ride promise"!