Despite the park hours not being published, you may still book your Fantasmic dinner package 90-days in advance of the date you plan to go. Since Fantasmic is shown every night, the only thing that is in doubt (based on the park hours) is the precise time at which you would view the show.
Park schedules (along with the parades & fireworks show times) are typically not published until about 60 days ahead of time. I'd guess they will publish the March hours at some point in late-December/early-January. Until then, you can get an idea of the historical hours by going to The DIS (wdwinfo.com) and putting in your dates in a utility there - it will show you the projected hours based on history.
I'm assuming that if there are two showings of Fantasmic, your reservation will automatically apply to the first showing, since that is the more difficult of the two to view.