Bakes,
9.3x64 cases are easy to find of late, Huntingtons has them...The 10.75x68 seems the best route and brass is presently available in good supply, now the problem is the reamer, no one has a reamer..

What I am thinking is perhaps the 8x68 is the proper case and one could use the 8x68 reamer in conjunction with a 404 Jefferys neck button or perhaps a 416 neck?????????? I hope, I hope...but I don't have a book of specs on the either the 10.75x68 or the 8x68, I will have to check this out.........

LuvSafari,
I don't think theCZ in 416 Rigby would be a good canidate for conversion, but it sure would make a nice 500....