It's not just the action - the magazine and bottom metal will also be a big part of the picture. By the time you're done, you may wish you had gotten a more suitable action to begin with. If you're having a rifle built up, put the project off a year, save some more money, and get one of the Granite Mountain actions that is actully suited for .505. If you can find an old magnum Mauser action, snag it, but it won't cost much if any less than a GM. Prechtel in Germany also sells modern Mauser actions, through NECG. Super actions.

A "sort of" Mauser option, really more of a bybrid Winchester/Mauser, is the Montana Rifleman actions. Available in a real double squarebridge configuration, easily suited to .505, for about 1/3 the price of most other options.

I think if you're thinking of building a rifle on a standard 98 Mauser, to save money, you'll be disappointed with the outcome. Better to do it right and have something that is a joy forever.