Question: I am looking for the 1968 French departments administrative boundaries. I've scoured the Internet but have had no luck.

Answer: 
JPEG from 1964: [Library of Congress](https://www.loc.gov/resource/g5831f.ct003434/)

You will have to georeference the image and digitise it yourself.
You can use "Historique des commune" from insee (<https://www.insee.fr/fr/information/2114819#titre-bloc-11>) which contains all modifications in french city since 1943.

It doesn't contain boundary, but can help to check the evolution of these administrative set.