I kinda wonder what kind of velocity you'd get from a 45/70 in a handgun.
Some things to consider:
A 45/70 is normally loaded with RIFLE powder. Rifle powder doesn't burn as fast as most pistol powders, so I'm thinking that most of the powder would burn AFTER the bullet leaves the barrel? If the barrel was long enough to utilize enough powder to give good velocity, the barrel would also be too long to really be effective in a close encounter. Might as well use a rifle (which ain't a bad idea, by the way).
As far as the pistol/rifle/shotgun for self defense against bears goes, I fit comfortably in the "I haven't been attacked so I don't know nuthin' 'bout it" category (thank goodness). That said, when I'm in bear/lion country I always have a handgun on me (for that matter, I usually have one on me any other time too...). I may or may not be able to get it out of the holster fast enough, but as a last resort I'd rather have that than nothing. Truthfully, if a bear/lion/anything else is so close that I can't get the handgun out, the rifle is going to be too long for me to use anyway so I still want the handgun on my hip.