First, the choice of a thumb hole stock is a personal one, and not to my likeing on a DGR, but as I say that's a personal thing. I have a Harry Lawson T/H stocked 243 win that I like very much, and I have seen several large bore rifles in the field with those stocks as well, so what ever floats your bass boat!

I think you can get along with a 22" barrel, but I would go 24". I certainly wouldn't go less that 22".

Your question in the other post about bolt mods, is correct, but is not a big job. The standard Mauser is made for the 30-06 rim size, as that is the size of the 8x57 MM,(.473"), but the 9.3X64 has a rim diameter of .492", .019" larger. However the Magnum bolt face is too large, at .532" the mod on the standard bolt face is a "no brainer" for a gunsmith. The biggist problem you will incounter is, the fact that the 9.3x64 is a rebated rim cartridge,meaning the head is larger(.504) than the rim, and the feeding will have to be tweeked a bit, so it will feed properly.

All in all, the project is not without some work, but worth the effort, IMO, if done properly,but that is also a personal opinion!