Thanks for all of the great info everyone,
Fortunately we were unable to find a steamship round with such short notice as it could have easily turned into a disaster. We ended up getting a 31lb beef clod (shoulder). That is about absolute max that a 55 gallon UDS can hold. It was an order of magnitude more difficult to keep the temps constant than the brisket I cooked the week before.
After a 17 hour peach wood smoke (tin foil during the last few hours) and 4 hour rest it turned out perfect. The whole party was completely stuffed and I have been having pulled bbq sandwiches from the frozen left overs for the last few weeks.