If you want something that will shift close to how that Shimano does, try to find a Spidel branded Simplex "Aerodynamic" SLJ6600. It won't be cheap, (could be 150 to 200 bucks for NOS) to complete the period group on your bike. Don't change anything, otherwise, cause what you have there is top notch stuff from Spidel and is period correct to the bike.
Congratulations on the great Gitane find!

Chombi