Brybenn..........You won't go wrong with a 300 mag Norma/Win/Wby but use 200 gn Nosler Parts, Swift A-Frames, or TSX. I know from experience that 180 gn Parts just aren't quite enough bullet for the Oryx or the Wildebeest. Last 2 trips I have taken only the 375 with 270 gn TSX, cause I'm getting lazy, and because of the ammo weight limitation on the plane. If you are considering taking the "significant other", who also is allowed 5Kgs of ammo on the plane, then by all means take 2 rifles...your 458 for the big boys and a good 300 for all the rest. Chances are you won't need 10 rounds of 458 for the buff, but you'll want to check your rifles once you arrive and this will burn up 3 or so if all arrives unmolested. Also you can buy 458 in SA very readily, so if you check and sight in with Win, or Rem or whatever ammo you can arrive with no ammo and have your PH grab you a box of what ever is available locally. This is a very practical solution to the ammo weight problem, and you can carry 5Kgs of your favorite 300 load.
If you're up for the recoil Brybenn, the 300 RUM is an absolutely devastating cartridge, with 200 gn bullets, and shoots like a lazer, it is truly a significant step up from the Norma/Win/Wby class of 300 Mags and you can pick up a decent rifle for around or even under a grand. My kids been killing stuff for years with his and it's a $600 700 SPS stainless, nothing fancy but it shoots several bullet weights under an inch (not all together, obviously) and he abuses the sh!t out of it, but hey it's stainless and plastic, who cares. Using 200 gn ABs it is an honest 700+ mtr rifle, and when it touches down sh!t dies !!! Just a thought.........I personally use my old 700 Stainless in 300 Wby, cause I've got indelible faith in her and I know how she shoots right out to 500+ mtrs, but this is after 25 years, 4 continents and God knows how many thousands of rounds and hundreds of animals. However the RUM still impresses me.