Here's the deal, if you swap out the stock, its not C&R anymore. You need to play the compliance game and switch out some foreign parts for US ones.
Keep in mind, you've now got a pistol grip and a folding stock. I don't recommend duckbill mags anyway but you will need to stay with the fixed magazine after switching to the STK6 or you'll have yourself an assault weapon.
If you order from SKSman.com or someone similar, make sure you get the compliance kit with the US parts.
My .02 cents, the STK6 looks like ass and I don't particularly care for ATI products (manufacture quality is all over the place.) If you're going to get anything, get the Monte Carlo stock by ATI because it does well to balance the gun. The STK6 is too light in the back and it makes the rifle feel awkward.