Rockwell's stocks have had a problem with their stocks cracking, toward the top of the grip where it meets the body of the action.
There's been some other issues going on as well.
Jim Rockwell Senior, is no longer longer a part of the company.
Jim Jr now owns and runs the company.
With the cartridge you are considering, I would want to put some pillars or reinforcements in that stock to ensure it doesn't go down on you.
I have several Rockwell stocks myself (they are beautiful), but they are all in cartridges that are 223 Remington case capacity or less.
A McMillan Stock, would have to have some modification/lengthened to make it work.
There is a prototype out of an aluminum center grip chassis (AR grip) for the XP, and I am sure one will be made for the 700 short action as well. I don't see that being available until sometime after the first of the year at the earliest.
Personally, I would not be real comfortable comfortable with ordering a Rockwell Stock right now. I hope I am wrong on this.