If installed correctly, a G&H side mount will handle the recoil of an anti-tank gun. I have one on a custom .375 and a .318 WR AE. They are far more durable than normal rings and mounts held by tiny top mounted screws. Installed correctly, the mounting screws on the side should not be visible, but I think you will be fine with how this one was done.
I would not trust that old Weaver on a rifle of that chambering.
Shoot it quite a bit and be honest with yourself with respect to recoil. The seller does not give us its weight, but it looks rather light. A .458 can be a difficult and unnecessary transition for a deer hunter. I actually prefer a .375 for buffalo rather than drag around a 40 something. So, spend the time to master it.
I would not trust that old Weaver on a rifle of that chambering.
Shoot it quite a bit and be honest with yourself with respect to recoil. The seller does not give us its weight, but it looks rather light. A .458 can be a difficult and unnecessary transition for a deer hunter. I actually prefer a .375 for buffalo rather than drag around a 40 something. So, spend the time to master it.