You would obviously need to pre cook all of the food and finish on the grill and in ovens. A smoker should only be there for back up. The entire running out of food idea is not a good one. You will not be invited back next year if you cannot support the crowds, they do not want to hear complaints that they came there for bbq and the vendor had none. Remember, there are plenty other vendors out there. You have your foot in the door at a great event, DO NOT RUN OUT OF FOOD.
Since this is a one day event for seven hours. Figure out what your staff can pump out an hour and go by that. I always figure the first hour to be 1/3 of what peak hours will bring.
Oh and as far as the dollar sign go, that is what it is all about! You can be successful and still have a good name. Good luck, from what I hear and read about you, you can pump out good stuff.