I don't know where you're having dinner. There's a good chance
a: It'll take you a while to get into the park. There might be people coming in just to get dinner, and/or to see Illuminations. You may even have trouble making your 7:40 reservation. You probably will, but there is a chance.
b: even if you make your 7:40 time, the park might be crowded enough that there will be a wait to get your table.
c: At that point, they might rush you out the door after eating, but depending on how long it takes to serve you, you might not have time to do much of anything anyway.
Having warned you of all that, there are some e-ticket Future World attractions that should still be open, like Soarin', Test Track & Mission: Space. If it's at all possible, I'd suggest trying to get a Fastpass on the way to your dinner reservations, but don't count on it.
Please understand, I'm not trying to dump on your plans, just warning you it might be a very busy night, and I'd suggest, instead of busting your own keister to jam as much fun in that last half-hour, to leisurely enjoy your meal, the sights & sounds and the all-around holiday vibe. It'll probably make for nicer memories than a mad dash on a full stomach to M:E.