Sarg,
Sounds like you are building a new back up rifle?
The Ruger you describe should be a good set up.
Regarding feed with the .458, a lot depends on the type of projectile you intend to use.
Spitzer styles like Barnes X are usually easier to get to feed well than the blunt round nose soft points like Hornady etc.
To get a 458 feeding well it is usually necessary to adjust/alter the mag box, follower and spring, ramp, and sometimes the rails.
The slickest feeding .458 Ive ever handled was one of the recent production CRF Winchester 70's, which had a recess machined into the mag follower that allowed the rear end of the case to align itself with the chamber.
I'm sure with some fiddling you would get a Ruger to feed well in 458.