Welcome aboard, SmokingEagle - a prime is one heck of a way to get our attention, and a great introduction.
I too will be smoking a large prime for the first time at the end of this month. Although the temps are great.. they don't really help with gauging the time do they.?!
My notes from others on this forum for preparing for my smoke are as follows:
6lb prime at 225 for a 135(rare) roast ~ 5 hours.
8lb prime at 250 for a 145(med-rare) roast ~ 7-8 hours.
I'm going to kick mine up to 275 and shoot for around a 5-6 hour smoke time, using hickory, for a 10 pounder. I'm thinking that the ends will be on the well done side, and the middle exactly the way I like it.
Good luck to ya, make sure and post pictures!