At least the 256WM can be formed relatively easily from an available case. That darn 25-20SS is its own entity nowdays (and even though it is listed by a number of vendors it MAY not be on stock but BO).
If I wanted or needed that case capacity above the std 25-20 case I think I woud go 256WM. That said, Im now playing with a 25-35 with a faster than the old 1:14 twist (so it will stabilize 100+ gr. bullets) and it is showing some real promise. I can load down to 25-20 or 256 and up to, in a strong action, some 2400fps or more. (zippy for me!) What a cool 'small bore express'. With these options it kind of makes the 256 a ' why bother?' in a custom barrel. A 257 Roberts would have more performance too but I like rimmed cases in a sgl shot.