Plenty of people use the 6mm PPC on the animals you are talking about and they are using bullets around the 68 grain mark at 3200 or so.

As to alternatives to the 22 Savage I would not be worried about pressure because you would be relaoding. Factory ammo for 225 would be real hard to come by and will be loaded to full pressure with 55 grainers.

Back in the mid 60s I used a 219 Zipper Improved on a Martini, as did my father and Don Black, a gunsmith that was in Sydney and who made many of them. Shot pigs and roos without problems. These were loaded back because of the small barrel shank on the Martinis. The 219 Zipper Improved is the 225 Winchester but with a different rim diameter. The 225 has a rim diameter reduced to .47 so that it would fit the standar Model 70 bolt face.

As I best I can remember we used the 50 grain Hornady SX bullet.