Careful. I have a Gitane TdF with exactly the same issues as you. I am using a Simplex SX630 (substantial all-metal upgrade to the SX610) rear derailleur that works with the old Simplex dropout. It is mounted using a bolt inserted through the back of the derailleur hanger.

The derailleur barely struggles to cover the width of a 6-speed cogset. If you were to mount this derailleur, or several other vintage derailleurs that I can think, of to an adapter as shown in the pic, then it would not be able to reach to the bigger cogs.

More modern derailleurs, designed for the longer travel of 8+ speed operation, should work with the adapter.