John, I have a couple cartridges based off the 243 WSSM. A regular 6.5 WSSM and a 6.5 WSSM Ackley. I really like both. All I do is neck up the 243 WSSM brass to .264 and then turn down the necks, A LONG WAYS. New brass is around .020". I don't remember the exact number, but I turn down to .013" to .014". I do it in 2 steps. I use Redding 6.5/243 WSSM dies for the regular 6.5 WSSM and Whidden dies for the 6.5 WSSM Ackley.
The 6.5 WSSM is my deer rifle and I shoot 140 Berger HVLDs at ~2860 fps. I know I can go quite a bit higher, but they shoot fantastic at that speed. This one uses a 26", 8-twist Bartlein. This Summer I'm going to do some more load workup on it and see if I can't find equal accuracy at a higher velocity.
The 6.5 WSSM Ackley was for F-Class. I had a reamer built for the 155 Cauterucios. They shot very well at 1,000 yards. My fire-forming load was 44.2 gr. H4350 and CCI 250 primers at ~2765 fps. My formed case load was 48.8 gr. H4831SC and CCI 250 primers at ~2845 fps. I was hoping for a little more velocity, but I couldn't get them going much over 2900 fps before I saw pressure signs. I'd still be shooting them today, but Bob passed away, and I don't have enough left for a match. With a .350 free bore I can't find another bullet that will work in it. I thought Berger was coming out with a 156, or something like it, but I haven't heard anything on that for a while. This one uses a 30", 7.25-twist Kreiger.
Finding brass can be hard. For a cartridge that nobody seems to want to touch, when the brass does get made, it isn't on the shelves for very long. The case is like a tank. I can't imagine how that brass wouldn't last you a lifetime, if you treat it right.
Whatever you do, don't go with a no-turn neck. You will not be happy if you're looking for great accuracy.