Daryl, 6,5/8x68 brass is easy to get in krautland but ist more expensive than PMP or Remington 458 win mag brass. The 6,5x68 have a base dia. of 13,30mm when the the 458 win have an dia. of 13,03mm max. after removing the belt on the lathe. The maximum dia. for the 11,2x60 Schueler is 13,05mm and so you see whats the best choice.
I dont like the trouble with necking up such a small neck, you have to aneal and working in many steps and in the end you have spliting cases.
The 6,5x68 brass is thick enough that you can lathe turned it to 13,00mm . So long as 458 win mag brass is available( the next 100 years?) will stay with it.Only 300 win mag brass , you find it sometimes here on the range once fired, will see a fire form load.

Tinker, asking myself what for a needle fire rifle this was, maybe for the english Needham cartridge? You see now what I mean with the upside down Paradox system. I havn’t see it but know that Dreyse build also double shotguns/ double rifles with two of this chamber inserts. This idea comes from tests for converting old smooth bore military muzzle loaders around 1845 and is much older than the paradox gun. Dreyse use this till 1895 when Rheinmetall buy the firm