Not sure of the S&W adjustable stock but I do know for a Magpul PRS all you need is an A2 buffer tube and the spacer that S&W makes just for tha Magpul PRS. Call S&Wcustomer service and order spacer # 432270000. Cost is about 14.00 bucks if I remember shipped to your door.
With this spacer you can use your stock spring and buffer. No other parts needed.
Now the bad news...it will take at least a month to get the spacer if your lucky.
Also check out www.grabagun.com for good deals on accessories.