The easiest way to get to the Magic Kingdom from MGM Studios is by bus. Just walk out front to the bus stop marked MK, and hopefully catch the first bus. You're going to be pushing it time wise though. Fantasmic lasts almost 30 min. and you probably won't be able to fight your way up front until at least 7:15 pm. Let's say you have to wait 15 min. for the bus and the ride takes about 15 minutes. Then you have to walk through security into the MK along with thousands of other people, and make it before 8 pm. It can be done, but it doesn't sound very enjoyable!
Do they show SpectroMagic twice that night? If so, you can make it in time to catch the fireworks, and wait to see the 2nd SpectroMagic, which is usually not as crowded. Whatever you decide, good luck!