Re:
Shot in stays - the frame design is pretty similar from 1983 to 1990. The internal routing for the rear brake was out through the top of the top tube in 83/84
and was routed out through the bottom of the top tube after 84,
The tubing changed as well, the most sought after bikes had Columbus SL tubing in 1983/84, after that Reynolds 531, 753 and later Columbus SLX and Super Vitus tubing was used on the higher end bikes.
There is a Gitane facebook group - well worth having a look if you have an interest in the Lemond/Hinault/Fignon years. That is where most of my info is coming from I have to admit!!