Hi Trex,

Hmm, difficult choice, since all of your rifles make nice combinations!

Two thoughts from my side (3 safaris, 4th one in planning, one buff down):

If your second rifle should also back-up the first one in case of a failure or loss, then I would backup the .458 w/ either the .375 (that will do fine on all the plainsgame you mentioned) or the .338 - which has the disadvantage that it is definetly not ideal for buffalo (but might just do, if you PH allows and your careful...)
(Note: Depending on the country you're hunting in, it also might not be legal on good ol' mbongo...)

Also, for back-up purposes maybe you want to check w/ your PH what client-rifle he has available. Chances are that there is a working .375 in the camp anyway.
(Note: it might not be possible for you to bring ammo for this rifle, if you don't have a rifle in the same caliber (e.g. .375) with you on the trip!

So, in a nutshell:
My recommondation would be .458 with either the .375 or the .338WM.

Regarding the .300 Weatherby check w/ your outfitter, since it is very seldom in africa that the add. range of the Weatherby's is needed, since most hunting is done in the bushfield w/ typical distances below 200m. Also - although it can and has been done - I would not recommend any .300 on eland (remember, those beautiful beasts weigh more than a grown-up buffalo bull!)

So, whereabouts are you heading in africa, some details of the planned trip would be interesting!

Enjoy, 500NE

PS: If you get your eland, absolutely insist on eland steaks for braai (barbecue)! I have yet to eat better meat!