Good advice so far. One thing I'll add is be aware of how much fat will be dripping on the fire and thus burning off when cooking that much meat. You can lessen this by running a diffuser/catch pan of some sort below the cook grate(like a pizza pan or somethin). I did two 9# butts last weekend on mine without a diffuser and the white smoke from drippings got a little heavy toward the end of the cook. Flavor was good, but I think it would've been better without that "fat smoke". Good luck.
Okie Joe offset, UDS, Akorn, Weber Kettles, I'm JD.