I just set back the barrel and recut the chamber on a friend's military '98 Mauser chambered in .257 Wby Mag. That would be another option for your Mauser. If it's a standard size bolt face, it will have to be opened up and the extractor carefully fitted. The .240 should work without any bolt face or extractor work. However, if it's a commercial Mauser with the magnum bolt face, the .240 case head is too small and the .257 is just right. The .257 has a modest advantage in velocity with 100 gr bullets, but Bb has proved that the .240 is very effective also. It would be a hard choice for me, but I always tend toward larger calibers and unfortunately toward doing everything the hard way. I think the tie-breaker for me would be if I had an extra .243 barrel or .257 barrel. That would probably do it.
Large ring Mausers are fine actions (most of them), reliable and accurate.
Sounds like an excellent project. Tom
Texas State Rifle Association Life Member
NRA Endowment Life Member
A big fast bullet will beat a little fast bullet every time