You are going to need a cartridge that can deliver enough energy at 450 yds. YES, sometimes they can be had closer and many times people get lucky with lesser cartridges. But I personally believe luck favors the prepared....so be prepared to shoot far and to hit them HARD. these animals are very very tough. And even if it is a kill shot, he might run 40 yds and you never recover the animal because the terrain is so unforgiving.
Shoot what ever you shoot the best but try hard to shoot something deserving of these animals, they are an African species so, shoot a little far forward. DO NOT SHOOT BEHIND THE SHOULDER. Try a high shoulder shot if you can. To anchor them down, other wise.....you'll be disapointed.