IronBuck
Yes the early Mk II Rugers can be converted to CRF quite easily and I've seen a few that have been done.
I agree with mickey that the 458 is a very good cartridge.
Bear in mind that the 470 NE pushed a 500 gn bullet at 2125 fps and this cartridge has a very good reputation.
The interesting point here is that the 2125 fps was from a 31 inch barrel so 470's with 24 or 26 inch barrels are running well under that original velocity and they still get the job done.
I,ve played with the 458 a bit and there are a few things I learnt the hard way.
Usually a fair amount of work is needed to get the feed reliable.
That big blunt 500 gn bullet is almost the same diameter as the chamber and wont find its way in as easy as a spitzer bullet in 300 mag.
Also the bullets in the magazine cop a real pounding from recoil and when using softs the exposed lead gets flattened out and this can hinder the feed also.
Don't know what stock you intend to use but the Ruger has a small recoil lug on the action and in hard kickers the wood usually breaks behind the lug unless the area has been strengthened and epoxy bedded properly.
458's with timber stocks really need a second lug on the barrel and bedded to the forend.

The Ruger scope mounts are a good strong design and will handle the 458 no worries but can benefit from a bit of fine tuning.

The old MK I Ruger 77 factory 458's were fitted with a steel floorplate and trigger guard and these were available for sale and could be fitted as a replacement to the cr*p metal ones.
This may also apply to the MK II's and can add a little extra strength and weight where it's needed.

Yeah, go for it I reckon.