Re: What dies to buy for a new build!
I can't address the issue regarding potential differences between the 7 SAUM and the 300 SAUM. What I can speak to is necking down brass in multiple stages. I think you already have a good bead on this one.
I shoot a 277 Allen Magnum, which is based on the 338 Lapua Magnum case. In order to make the brass usable for my 277 AM, I have to neck down the brass in stages. If memory serves me correctly, I first neck from .338 down to .323; then down to .308; then down to .284 and then finally down to .277.
Unless there is a difference in case design between the 300 and 7 SAUM, you should be fine using either one for your 6.5. I would aneal the case neck first and then neck down in stages.