Do you know whether you have been assigned to the early or late Fantasmic showings? I believe later dinner reservations are automatically assigned to the late show. The first show plays at 9 pm, the second one at 10:30 pm. So, either way I don't think that that your dinner is too late.
When I've done the BB Fantasmic package in the past I usually show up to the viewing area no more than 30 minutes before show time, sometimes only 15 minutes before, and have still gotten great seats. I can't promise the same will be true for you, but I don't think you need to be first in line an hour ahead of time to get a good spot. In fact, I know some people like to wait a bit before entering the viewing area, because they prefer to sit a little further back so that the railing is not in their line of sight.
So, even if you are assigned to the early show for some reason, you should be right on schedule to enjoy your meal for 60-90 minutes, then walk right over to the viewing area and take your seats.