Good luck with your new no.1 in 450/400!
I must tell you, that caliber was once(before the .375h@h came about), THE all-round caliber for Africa.
Now, that does not mean it is ONLY for Africa.
That would be an outstanding cartridge for Grizzly, and anything else in America-except small species like goats and small deer-but hey, this is an insignificant issue, because I've seen 22-250's tear up more meat on a deer than a .375. One has to wonder, where people get the idea that faster is always better.
I think NE450no.2 hunts wild hogs with his double, and comes out clean I imagine; so no, there is absolutely nothing wrong with your 450/400 here in America, and I figure, that is how Ruger is planning it too. I'll be hunting hogs with my Lott also.
And I think you will find that most of the dangerous game rifles that exist today, are used more on tamer species than anything else, simply as a matter of interest on the part of the owner, skill building, and mostly just feasibility, as not everyone can afford a trip all the time. It helps doing this, after all, because it may have to be taken against something meaner in the future, and anyway, it's just plain fun, and a lot more enjoyable that using new-fangled ammo and guns.
I used my .450 3 1/4" on hogs for a short time, with "light-cordite" loads with lighter bullets. Same thing I'm going to do with the Lott.
I would not hesitate to use it for Grizzly at all, in fact it is spot-on for that work-with full-power loads.
Although, and I must say this, you may be hampered by the single-shot capability, in a tight spot. BUT, hopefully that will never come about.
Study the ballistics, and drop in bullet at about 150-200 yds.
This will get stuck in your head. Closer than that you should be spot on. Thats just at the top of my head-so don't take it as gospel-I don't remember the ballistics.
I'd use it for all big-game here in America, regardless of anyone elses opinion. It is an excellent caliber.
In a single shot, with a close shot(50-75yds) at ol-Griz, be sure you have a man with a back-up rifle-they can close distance faster than they look on tv.
For all other game here it's fine as is.
FINE cartridge. With the right bullet it will take anything in America-and you can load it accordingly(you will be hand-loading, as I imagine the factory ammo (hornady?) is going to be pretty expensive), since it's a single shot-unlike a double, which would have to regulate with a certain load.
It only faded somewhat with the advent of the .375h@h.
There are no flies on it-it was just asleep for a little while.
I'd love to have a double in this chambering for big bears. But I might have to settle for one of those no.1's myself.
Good luck!