It's worth noting that the Air France nonstop CDG-DFW doesn't operate on Sept 18th.

For the outbound I checked both the 17th and 19th for you. No availability on the 17th but the flight on Sept 19th (AF 158) is showing Z6 so thats likely your flight.

For the return ( AF 159): Dec. 17 and 18 are both showing Z7. The flight on the 19th is Z0. Both return dates price out the same on a round trip booking and you can book through Delta (codeshare) or Air France.

As you might have read on other threads Agents are not always well versed in GUC. It's important to note that Z class availability for the upgrade needs to be searched for by the Agent under the Air France flight numbers (operating carrier). Agents will often search the codeshare flt which typically shows unavailable. I would recommend calling Delta Mon-Friday during US business hours to apply the certificates (weekend agents are typically challenged)

Good luck!