There is also the 9.3x57 (.366") wich is a common European large game cartridge, especially in Sweden an Norway for moose. This can be formed from 7.9mm Mauser brass. A very old cartridge is the 9.5X57 (.375") Mannlicher cartridge, which still uses the same basic case. It was made obsolete by the .375 H&H Magnum, but still would be fun for a sporter project. Neither of these would require magazine alterations, but the feed rails may need work.

The .35 Whelan is a 30-06 legnth cartridge, and will work in a 98K action. However, some factory rounds may be too long to fit in the magazine box. Just try loading some longer 30-06 cartridges (like surplus ammo) in your magazine, and you will see what I mean. However, the work needed to make the slightly longer rounds function properly may not be prohibitively expensive.

Good luck.