When you change buffer tubes, you will still use the OEM spring, unless you get one of those fancy JP Enterprise captured spring/buffer assemblies.

If you order the $10ish 2" spacer from S&W, you can use the OEM carbine size buffer. You can also increase the weight of the stock to further curb felt recoil by using a heavier 2" stack of steel washers as a spacer instead.

Just a quick tip: if you can't get the castle nut to loosen without marring it with the wrench, put the lower receiver in the freezer for thirty minutes to allow the aluminum threads to contract. Working with the aluminum parts cold will result in less force needed to loosen the nut.