Hi Flybye,
LRRH is a lot of fun! I hope you'll join us.
As far as I know the t-shirts are the same for all the distances (they were last year). Also, I think that registering for a certain distance is just so the organizers can know what to expect as far as food goes (how much to have at each stop). I don't think it would be a big deal to go further than what you signed up to do.
BTW, I signed up for the 64 miler last year. It ended up really being almost 70 miles. The last few miles are a killer if you're not expecting them, so be forewarned. My ride time (not including food stops) was just under 4 hours.