You don't need a 338 for plains game a 300 is perfect and ammo is available every where in SA. Very little plains game hunting will be up close or in the thick so sights are just an annoying add on. I always take a spare scope all set up in identical rings and sighted in, in case of an accident and my scope gets damaged.
Bolt action for sure and practice right out to 400 mtrs if you can. Most places in SA get hunted up to 200 days a year and the game gets quite spooky, long range capability can really up your odds on good trophies. I personally use a 300 Wby in a 700 Rem stainless with a 3-9 Zeiss Diavari and 200 gn Parts or lately Accubonds. I've taken everything from 20 lb Suni to 2000 lb Eland with it and not found myself ever needing more. I personally like the extra reach and power of the 300s over the 308 or '06 but both are still a fine choice, if you're a little recoil shy, and have worked for many years on these hunts. Most PHs I know who carry a rifle on these hunts will likely be using either a 308 or '06, so what does that tell ya.