Kammerherren, I'm looking at the same rifle in the same chambering myself. I also noticed the apparent weight discrepancy on the Ruger Web site, which I find very curious. The Ruger No. 1s do seem to kick a little harder than other designs in the same caliber, in my experience. Nice thing is that a good Decellerator or Limbsaver recoil pad and perhaps a bit of extra weight in the stock bolt hole will soak up a lot of the recoil and allow you to fine-tune the rifle's balance. I've shot a No. 1 that was rechambered from .458 to .450 NE and found the recoil just tolerable, but I don't have the high threshhold for recoil that many other shooters seem to have. I think you'll find the .450-400 very easy to shoot. And of course you can change the performance/recoil level with handloading.
Good luck getting your hands on one.