I realize this is a very old thread but someone might read this today so here goes nothing!
The rifle is a good looking one, and well thought out for the cartridge. It is scoped approtietly as well in QD rings and with iron sights. So far this is a fine example of the hunting rifle for anything North America has to offer, ALMOST!
I have no idea what the rate of twist is in the barrel but I can see that the action is an older Ruger which are push feed
actions. That is OK in this case because the cartridge is basiclly as stated a 45-70 in a belted rimless case to make it easy to use in a bolt rifle and feed properly. The stock is a good one for North American wetter climates like Alaska, and Canada. This would make a fine woods rifle for moose, or black bear. However, for Grizz, or Alaskan coastal brown bear, this rifle would be OK if used over bait from a tree stand, but the PF action makes in unsuited for dangerous game from a position on the ground.
The bullet you are wanting to use is also not well suited to this cartridge, especially in close quarters with bite-backs. A quality 400 gr bullet is a far better choice IMO, for the 458X2" American, and exasorbates the problem found in the 458 Win Mag, of limited powder capacity for a 500gr bullet. With a well designed 400 gr bullet the rifle would be far more suitable for dangerous game in North America, and would work quite well for Lion on bait in Africa, as well. The 458 Win Mag is hard pressed to use a 500 gr bullet, because of the limited powder capacity, and this cartridge has even less capacity. the 400 gr bullet will be a better choice for this one!