That's an hour tighter than I would plan for that transfer. Doesn't look like you have a choice either; I checked the train schedules and that's your only option.

September 5th is a weekday; traffic can be a real crapshoot. If there's construction, the trip can take quite a bit longer than normal.

Are you familiar with Gare Montparnasse? It's huge, two separate terminals actually, so you want to make sure you go to the right one. If you purchased your tickets via trainline.eu, you'll get platform notification which can be a real help.