I have one in the works, it will be based on a Rem 700 long action. Found a "Very Limited Production" set of 6.5/300WSM Lee dies, and will also get a Redding 300WSM body die. Will be necking down 270WSM brass (only reducing the neck .013", so much easier than necking down 300WSM brass (which would take a .044" neck reduction).
Here is what Kevin (Board Sponsor, MCR Rifles) states about the dies....
The difference you're seeing from the 270 WSM to the 300WSM is from the smaller diameter neck coming down the shoulder angle. If you order the Redding body bump die you'll want to get a 6.5-300 WSM body bump. The body bump will resize the body and bump the shoulder back .001" - .002" when set correctly. If you got a 270 WSM body bump die you won't be bumping back the last .013" on the shoulder.
I also spoke with Redding, and they told me the bushing dies would not size the entire neck, because of the radius on the inside of the bushing (but I bet it would still work).