Morten,

300 H&H Flanged would be the most sensible choice, if it could be done (most likely it will be very simple convertion compared to larger caliber). After this gun been renovated you need a double suitable for thick skinned game

Managed to dig out some C.I.P data over the cartridges pressures

308 Win.
4150 PTmax= maximum average pressure (bar)
4773 PK 1,15 Pmax = maximum statistical individual pressure (bar)
5190 PE 1,25 Pmax = mean proof pressure (bar)

30 R Blaser.
4050 PTmax= maximum average pressure (bar)
4658 PK 1,15 Pmax = maximum statistical individual pressure (bar)
5060 PE 1,25 Pmax = mean proof pressure (bar)

30 Super Fl. H.& H.
3200 PTmax= maximum average pressure (bar)
3680 PK 1,15 Pmax = maximum statistical individual pressure (bar)
4000 PE 1,25 Pmax = mean proof pressure (bar)

303 British
3650 PTmax= maximum average pressure (bar)
4198 PK 1,15 Pmax = maximum statistical individual pressure (bar)
4560 PE 1,25 Pmax = mean proof pressure (bar)

375 Fl. Mag. N.E.
3250 PTmax= maximum average pressure (bar)
3738 PK 1,15 Pmax = maximum statistical individual pressure (bar)
4060 PE 1,25 Pmax = mean proof pressure (bar)

450/400 N.E. 3"
2800 PTmax= maximum average pressure (bar)
3220 PK 1,15 Pmax = maximum statistical individual pressure (bar)
3500 PE 1,25 Pmax = mean proof pressure (bar)

450/400 Mag.N.E. 3"1/4
2950 PTmax= maximum average pressure (bar)
3393 PK 1,15 Pmax = maximum statistical individual pressure (bar)
3690 PE 1,25 Pmax = mean proof pressure (bar)