Since you mentioned the 264 win mag I assume that one of your actions has a magnum bolt face. I would just go to the wsm case as the brass is very strong, and it is something different that not many guys have.
You are correct. The stevens action was originally a 300 win mag.
6.5 WSM is something I hadn't thought of. I've heard a lot of mixed reviews about WSMs (along the lines of 'gimmick' but some good things, too). Does anyone here have one to comment on performance? Is this a barrel burner or a more conservative chambering like the 260? I had a mind to make a real barrel burner and only load it really hot on special occasions. Since the WSM is so short, there would be a lot of room in the magazine for LONG bullets, which would be nice. Not sure how much bigger than 140gr I would want to go though without getting into a special twist bbl.
One thing I like about the 264 win mag is that not many people have one of those either, though it's harder to find brass since you cannot just neck down 270WSM. I like unusual calibers. Would the 6.5mm wsm still use a standard magnum bolt face? I notice the rim is a little thicker in the WSM case.
Is it necessary to use a magnum primer with the WSM? My thinking is that could effect accuracy, no?