Another suggestion, rebore barrel to 358 and open the action to accomodate the 350 Rigby rimless. If you go this route also have the action lapped, and checked over and re-hardenened.

I think sticking with 318 W-R is better even if the lining of the barrel will costs a few pretty pennies. It's a great looking rifle. Funny the stamps on the barrel says 8x60.