45 FRIGGIN MPH??? That is really moving, especially with all that weight! I guess having a slower friend go with you comes in handy here!
As for the bird shot, what if it was fired from a short barrel such as an 18 incher or 20 incher? I don't have too much experience with shotguns so I don't know what the expected patterns would be at varying distances with no chokes. I know most people think it spreads more than it actually does though.
If I was lugging a 12 gauge, I would think about bird shot (#4 maybe), followed with 000 Buck, then slugs. I better shoot fast or hoot early though. I doubt I could get 3 rounds off if the bear is closing in at 45 mph!
If we are talking about theoreticals here, I would carry a flame thrower. He'll be Smokey the Bear for sure! They we can feast on him afterwards!