I've killed 5 black bears on 7 hunts (all in Maine).
I agree with most of the sentiments here. MOST.
Your 45-70 is fine. Black bears don't require a magic bullet, or much more than the average whitetail.
That said: Black bears can range upwards of 500 pounds and, while not common, those bears take a bit more abuse then the average 150 or 175 pound bear. Yes, more people have been killed by deer than bears, BUT bears are more than equipped to take you on when wounded.
One of my best friends, a Maine guide, explained the difference between bear and deer to me 20 years ago on my first hunt. His deliniation between bear and deer hunting is that while deer are smart, that bears are "thinking animals". I believe this to be true, having tracked a few wounded ones (others, not mine, thankfully), and observed many others close-up.
I think going into a bear hunt with a degree or two more respect than you might impart to a whitetail deer is a wise option.
There IS something very primal about being close to a bear. In that way, it is most unlike deer hunting. You're hunting a hunter.
Enjoy your hunt!