450 Dakota
Find out if the rifle has .408 or .411 diameter barrels. Then you can order some ammo from www.superiorammo.com.
Get 400gr Woodleigh Softs and Solids. There ammo is very good and should give you some indication how the rifle shoots.
The 450/400 has plenty of power for any game uncluding buffalo and elephant. I have used my 450/400 3 1/4" to take deer, wild pigs, caribou, and black bear, I took it on a brown bear hunt but did not get a shot. Give us some more info on the rifle, ie bbl length, weight, the load stated on the bbl flats and which action it is built on. Pictures would enable the experts here to give you exact info.