Raimey,
Make cases for each of your 3 chambers and keep them separate. Then, you can neck size (partially size) to be easier on the hard-won cases. If you reduce the fireforming load from 11 grains Red Dot to the minimum necessary to form the cases, you might save some cases. The straight 6.5x70R cases shouldn't need much forming. Are you using a filler in the fireforming loads? Filler with a lighter load is easier on the cases than powder alone. There is no formula, you have to try and change the load, depending on the result.
If it were easy, it wouldn't be fun and you wouldn't learn nearly as much. This particular cartridge has fewer options for donor brass than any that I know of. If you can locate cases for the relatively modern Einstecklaufs chambered in 6x70R your job would be much easier and the cases would be better. I suggest you query the makers of these devices; they likely make few but will surely test and sight in some of the ones they do make. Also, gunsmiths that sight them in likely have a box containing misc. fired cases, possibly including what you are looking for, under their work bench or at the range. I found a considerable amount of useful brass from similar sources when I lived in Germany, but the 6x70 R was not in use then. Maybe some of your European friends would be willing to ask gunsmiths in their home area for cases.
Good Luck,
Mike
