Actually, instead of a clod (which is a confusing piece of meat to seperate into it's parts) try an inside round - this is used, often, as the roast been sliced on cafeteria lines. They are lean so you would not want to overcook them -- however, taking them to say 145 or so (check with the deli to see how rare he wants it) - most of the inside rounds I used to cook were around 17-20 pounds and unlike brisket they are not thin, flat pieces of meat.
The clod would work, too, but I'd break it down into the seperate pieces - otherwise you are looking a a truly large piece of meat. Now, my experience was with IBP primal cuts ordered in by the case -- this is how you'll want to get your orders -- check with SYSCO, GFS, USFoods, or other wholesalers for case lots -- Sam's or Costco would work, too.
Good luck. Just think of all the trials you'll need to run!! :D Hope your family and friends like bbq beef, a lot.
Oh yeah, the Bandera probably won't be big enough for these cuts.