Re: necking down .284 to 6.5x284
That's how it's done, though you'll probably have to neck turn.
I question why you don't just buy 6.5-284 brass outright?
The 6.5-284 and 284 phenomenon is strange, huh? The parent case was never popular but its offspring took off - to the extent you can barely even get straight 284 brass, and we're necking up "wildcat" brass to get back to the parent case... strange