OTL a fortification program was started after the 1938 exercises referred to in the Wiki article
https://en.wikipedia.org/wiki/André-Gaston_Prételat
That program was based on assorted existing contingent plans. Between a shortage of appropriate construction labor, that funds were not immediately available in the autumn of 1938, severe weather in the winter of 1939-40 completion was very slow. To get as far as they did the French Army was forced to use the Series B formations as construction labor from their mobilization in October 1939. That reduced their training time to as little as one day a week through the Winter and Spring of 1940. At that the fortifications that could be built were light compared to the CORF system that is usually labeled Maginot Line, and incomplete. More important is most of these new defenses were manned by stale and poorly trained reservists. That the good quality regiments and battalions were able to stall and seriously delay the enemy on their sectors, until outflanked and overrun shows what might have been had all the army spent the winter training vs digging bunker foundations.
To get to a properly manned fortification in this are you need a PoD much earlier than 1938, and it mostly involves the Chamber of Deputies and cabinets os of the 1930s be willing to raise taxes significantly or indebt the French government in a major way.