A 45-70 has more than enough power to drop a bear, even if your useing black powder, remember the 45-70-405 was a buffallo killer in the turn of the century, it still is in the year 2009. What type action are you useing to hunt bear? If its a lever gun then a quick follow up shot is no problem.
As for a back up 12 gauge......SLUGS!!!! and use a shot gun that has sights, I dont mean a brass bead either, I mean rifle sights or ghost ring sights. Williams makes some really nice Fire Sights that will really grab your eye fast in the woods in low light conditions. I'd stay away from Buck shot....its a BEAR, not a buck. I never heard of any dangerious game guides ever useing buck shot......Its always been a BIG BORE GUN. Even Howdah Pistols used a single large projectile to kill a tiger or lion if it tried to climb the back of an elephant and get into the Howdah Basket.......Hence the name Howdah Pistol.
Bird shot.........again its a BEAR, not a bird.
Good luck on your hunt.
Dont worry about recoil.....you aint gonna notice it if a bear is charge'n you, recoil will be the last thing on your mind.