Not the best choice, get the speed up to see knock down. We just hunted Grizzly and I watched a good member here fold one with a .300 Ultra to the chest. As I slowly gain experience on game over time I've come to an uncomfortable realisation; speed kills. Or at least it shocks.
And I don't mean 3,200fps impacts, though they're impressive, but rather bullets hitting their target above roughly 2,400fps. Don't know how I come to that number but it seems a natural delineation. Most conventional hunting cartridges impact around or above that, and it sure seems like that's where "shock effect" starts. This is not an easy admission for me as I've always been a moderate and heavy for caliber thinker, but experience is proving that a shaky assertion. Culling in Zimbabwe I've been lucky enough to watch multiple rifles work, from .30-06, to .300 mag, to .375. By far the most impressive and decisive kills belonged to the faster loads. I had specially loaded "culling loads" I'd made of heavy .375 bullets at about the velocity your 300gr will move, literally the same weight and speed range you're planning, and they were not terribly effective. Everything died, but not as quickly or as close to where they were shot as with the quicker loads.
Grizzlies are exciting, but "soft" animals. While I've just started doing it I guide hunts for them now and am peppered with "which gun" questions from prospective clients. I have to admit a.30-06, .300, or .338 mag seems near ideal in today's light. .45-70 would be low on my preference list- though it will certainly kill them. It is far from the best tool for the job though. .45-70 isn't the best tool for any hunting though mind you with its trajectory, so I'd say pack any conventional hunting rifle with confidence. .270, .30-06, .300, yada yada yada. Good hunting to you! Where does your mainland quest take you, and after what species? I'd base the gun on that.
Post Script: I should ask how fast you load those 300's in your .45-70 and what they're flying out of.