I am not especially warm-blooded, so take that with a grain of salt.
Highs in the 70's means that early morning temps will be 55-ish, and it will be in the low 50's at fireworks/Fantasmic time.
And although I probably wouldn't wear a coat at 50 degrees to run from my car into Wal-Mart, standing still in line or sitting on those (concrete?) benches at Fantasmic is a whole different ball game.
I would take long pants, short sleeved shirts, and sweatshirts for layering (tied around waist or stuffed into a locker during the day).
For me, shorts would be a waste of luggage space.