Well, first of all, you'll probably have to plan on Mythos for lunch. If the park closes at 6, Mythos can close as early as 3:30.
A day or two in advance should be plenty for making ressies. The number is 407-224-4534.
If you'd rather not take time away from the rides, you can eat at Mythos on a day when you don't have park tickets by using the "Diner's Pass".
Make a reservation, then go to Guest Services and tell them you have a ressie at Mythos. They'll take an impression of your credit card. if you and your party are back within two hours, you get the slip back. Otehrwise they charge your card for admission for everyone in your party. if you're running late at Mythos (and you might) let your server know and they'll call Guest Services and cut you some slack.