George,
Let's start from the basics to make the project viable, where will you get a 425WR reamer from in Australia? If you have to buy one, then better to put the money towards a 404Jeffery.

Better yet ... send it to TSE and rechamber/barrel for the 10.75x68Mauser. A 400gr will still wing its way at 2100fps or original Kynoch 404Jeffery ballistics.
Cheers...
Con