Arthur,
Couple points: The first is, a number of people have had great success with the 6-6.5x47 Lapua. Climate may play a role with the SR-size pocket, but that's a guess. And, you could always drill flash holes, or as another person pointed out, probably make cases from the Norma 6XC brass. (I'd check that one to be sure). Point is, it may be premature to jump ship.
But if not, moving away, the 6XC is almost exactly the same capacity as the 6-6.5/47. Norma brass, LR primer. I'd read German Salazar's writeup on his blog. Custom dies may be needed, but they're usually a good idea anyway.
As is a 6 on the 6.5X454 M-S case. I bought 500 pieces from Black Hills a few years ago, when they were cheap. May still be cheap. I changed things a bit, but with my variation on the 6.5x54, I get a case full capacity of 52 grains water. It shoots just fine. The .243 Winchester is, as I remember, 54 grains water. Don't quote me.
Greg Culpepper found a good deal on Carcano cases, and made essentially a 2-inch PPC. It shot OK at 1,000 yards, but only that. There are a number of reasons it might have been middle-of-the pack, most likely the particular barrel.
Long ago -- mid 1980s -- Clarence Hammonds simply ran his 6 PPC reamer in long, and using the Arisaka case, created the "Rampage." He reported it shot great, but of course, he wasn't using long-range bullets. There is a report somewhere in Precision Shooting.
The Arisaka, Carcano, and 6.5x54 M-S can all be though of as slightly larger/smaller variations of a 2-inch PPC. The Arisaka is semi-rimmed, so would take a boltface larger than a PPC. I use a .308 boltface with the M-S case. Boltface is not an issue, unless you have a PPC boltface and choose the Arisaka.