Hi Tom
I have a Leupold VX-6 1-6 x 24 on my Winchester Model 70 in .416 Rem Mag. The rifle only has about 100 rounds through it since I changed scopes, but the scope is holding up fine. The only problem I see with it is the size and weight, but I do like the 30mm tube, and the scope has excellent field of view. Heavy scopes can certainly test your mounts, I am using Talley Lever Lock QDs and really like them. The scope has around 4" of eye relief, but it is non-critical and has been more than enough for the .416, so you should be ok on a .458 Win Mag. The German Number 4 reticle that mine has is a bit thinner than I normally like but I am used to it now and have hunted with the rifle and scope a bit and I didn't notice. The one power on the bottom end is nice is close bush, and the six power is more than enough for any shot you are ever likely to take with a .458, in my opinion. I know a guy who has the same scope on a rifle chambered in .450 Rigby and it has held up fine. The price is not cheap even compared to say a VX-3 1.5-5 x 20 (which a lot of people also like for big game rifles) but the VX-6 is a lot of scope for the money if you want to spend that much (over $1,100 in Australia), or don't want to spend as much as you will need to for a Z6 Swarovski etc. The people I know with Z6 Swarovskis like them very much, I have just never been able to bring myself to part with that much cash for a scope.
My back up scope for the .416 is an old Leupold VX-II 1-4 x 20 that has a couple of hundred rounds of .416 under its belt and many more full power 45/70 handloads out of a Marlin guide gun. That scope has never lost zero or given any trouble, even though the VX-II series is not meant to be as recoil resistant as the VX-III line, as you probably know, due to the VX-II only having a single erector spring compared to two in the VX-III. I might have just got lucky with it.
Another scope to consider if you don't mind a fixed power is the little Leupold Ultralight FX-II 2.5x20 scope, I have used one of these and they are tough and have around 5" of eye relief, which is always nice on a big game rifle.
I certainly agree with the opinion that has been expressed above, that reliability is more important than optical quality for a scope on a big game rifle.
I know a lot of people will tell you that open sights are the go for .458 class and above rifles, and it is all personal preference, but with my ageing eyes I just can't shoot with open sights like when I was a kid, you might be fine.
Best regards
Glenn