The 338-06 is just a barrel away and a great caliber. A 338 mag would need a barrel, bolt head, and magazine. I don't think a 416 Rigby is going to happen, but you could do a 416 Ruger with a barrel, bolt head, and magazine swap. The Ruger is right there with factory Rigby ballistics. If you run the Rigby a little hotter it will out run the Ruger by a pretty good amount.

Andrew