For a large cook like that I plan 16 hours from meat on the smoke time, so fire up the pit 16:45-17:00 before serving.
I would not expect it to cook that long, but I MUCH prefer buffer and hold time to rushing. When you pack to the gills, you have the cold meat factor, less airflow around each one, more moisture squeezed out causing evaporative cooling, more door open time and other factors than can slow you down. Add it a few stubborn butts that might take longer etc. and I expect 12 hours at 275-300 to get them done. That leaves 4 hours of hold time and pulling time. If they are all done in 8 hours, no big deal, you have 8 hours to hold and pull. If they take a few hours longer, you may be rushed on the pulling (recruit volunteers!) but you will still serve on time.
My other tip for big cooks. Go to harbor freight and get a pack or two of the white cotton gloves and a box of the heavy duty black nitrile gloves. If you give volunteers gloves to protect their hands from heat and grease, they will be more willing to help and they will be able to pull much faster.