I would have to agree with Chuck375 and Dorleac that the best CLASSIC rounds for a Mauser 98 and the ONLY ones I would consider for a high end custom rifle would be the Jeffrey duo mentioned above.
If I were to rechamber a .458 Win Mag I would consider the .458AR (Accurate Reloading) based on a blown out and shortened .404 case or the .450 A-Square pictured above based on a 2.5" improved Rigby case. The .375 Dakota and .375 Ruger would be better choices than the .375H&H for a 98.