As an Alaska hunting guide myself, I can say that you'll have tough time finding a guide up here who'll embrace the idea of a client bringing a .45-70. That's not to say that there's probably a couple or maybe a few who'll allow it, but they'd likely be much happier if you brought something that shoots considerably more flat.
And I'd pass on the various .300 magnums. I'd also pass on the. 338 Winchester Magnum (why physically abuse yourself with that?). Go with a .375 Holland & Holland or .375 Ruger. Fabulous trajectory - about like an .06 with 200 grain bullet - and especially when considering all that bullet weight and delivered kinetic energy.