Cooking a big bird can lead to an overly dry bird since it is hard to get all sections done adequately. Have you thought about cooking two birds? One can be a "presentation" bird and the other one you can cook the night before, slice, and put on a serving plate which makes it easier for serving large crowds. It is going to be such a large crowd you will likely have a buffet set up and you don't want people hacking away on a big bird cutting off literally more than they can chew.