I prefer to neck up, rather than neck down, if possible, so I used Winchester/Olin 243 brass to make by 25 Souper cartridges.

Everybody and their Mother make 25-284 reloading stuff. I use Winchester/Olin 284 brass to make all of my 284-based wildcat cartridges. You could use Norma 6.5-284 brass also, but I haven't, so can't comment on it.

Jeff