Eddybo, just go with a 6x284, you will love it. All you have to do is neck down the 6.5x284 brass and your ready to reload. I am putting one togather right know, and I am going to develope it around the 105grn and 107grn bullets.
If your dead set on using one of the WSM cases, go with the 300-WSM and neck it down to 7mm. Norma is making the 300-WSM brass, so you will have great brass.
Let us know what you decide. What kind of specs are you going to use for your project.