Several guys that I know who go to alaska on a regular basis have .45-70. Never needed to use them (thank goodness!), but they don't feel comfortable with anything less. And somehow, methinks that if you ever had to use it, you'd want one heckuva powerful round flying out the end of that barrel, and .45-70 should do the trick.
This is one of those "get what you know will work instances, not a "what could suffice to kill that charging [fill in the blank]".
If you're going to take a .454, it'd better be a revolver, and should not substitute for your main weapon. .45-70 is a heavier bullet that is going faster--which is a combo you'll need, if you wind up having to use it. Frankly, where I work, if you wanted to buy a .454 levergun for potential alaskan defense, I couldn't sell you one in good conscience, and would push you to buy the .45-70 (and no, I don't get a commission off of it).
Jackal is right, though--a 12 gauge slug is the best option, but the .45-70 is a close second. That, and it gives you a good excuse to buy a different rifle. If you do buy it, though, a suggestion--get a good gunsmith to make it a "takedown" rifle; several of the guys I mentioned in the first lines have done that, and it helps when cargo dimensions must be limited (they ususally get to their fishing holes in little civilian planes, and the space saved is, literally, a "life-saver"). HTH, and good luck.
