Asked the same question in this thread:
http://450bushmaster.net/viewtopic.php?f=3&t=15559 and got no reliable answer.
Therefore i asked magpul directly. This is the conversation:
My question:
Hello,
will the Hunter American Stock fit all Ruger American short actions, explicitly including the Ruger American Ranch in .450 Bushmaster? (And if so, what magazin would you recommend for that application?)
Their answer:
Thank you for contacting us. Our Hunter American stock will fit any of the Ruger American short action centerfire rifles. You can use the mag that comes with the Hunter American, it will feed as good if not a little better then the mag that comes with the factory stock. Currently i have my RARR back at the dealer/gunsmith. When the feeding issue persists, i will definitely try the magpul stock.