Greetings Nigel!
How exciting to be visiting the Walt Disney World Resort in October!
50's Prime Time Café is my family's favorite place to dine when visiting Disney's Hollywood Studios. You just can't beat the fun antics and hearty meals that this popular dining location is known for.
Although you are welcome to check in for your Advanced Dining Reservation earlier than 6:40 p.m., you are not guaranteed to be seated early. Because seating is limited for Fantasmic!, I usually try to get in line at least 45 minutes prior to the start of the show. To be honest, I'm afraid you are going to be cutting it too close to eat at 6:40 p.m. and try to make the 7:30 p.m. show.
My recommendation is to try to find an earlier Advanced Dining Reservation for 50's Prime Time Café so that you can take your time and fully enjoy the experience. This would also ensure that you make it in time for Fantasmic!, which is a "must see" in my personal opinion. If you will be spending all day at Disney's Hollywood Studios, you may want to consider booking an Advanced Dining Reservation for lunch and then enjoy a Quick-Service dinner in order to have more flexibility before the show.
I hope this information is helpful to you. Please don't hesitate to visit us again with any additional questions that you may have. Best wishes for a magical trip!
Happy Planning,
Jodie