Quoted:
The first one you showed a link to just replaces your standard stock.  It's made to give your rifle the "look" of a triangle folder at a glance without any machining.  It could almost pass with the stock open but it will look ridiculous while closed.  Basically, a big chunk of plastic and metal will be sticking out the back of the receiver when you close it by two to three inches.  I've never seen one in front of me but I've seen pictures.  It's pretty gay.  

On the other hand, the poster that made the last comment hit it on the head.  You can't do this job yourself.  You'd need to send it to someone and pay them to do it.  The rear trunnion block and tang would need to be removed, the receiver cut (very exacting if it's to fit correctly), and the folding trunion and assembly riveted into place.  Don't forget the front latch/catch needs to be installed as well.  All in all, it'll cost you some money.  But it will be well worth it.  It's not too hard to find someone that can do this correctly.  But it's not going to be easy to find the side-folder.  Not the complete kit, anyhow.  They're around but you have to really look for them.  Currently, I believe it's DPH that's supposed to have some Taiwanese copies coming in but I don't trust any aftermarket parts from Asia.  K-Var will probably never get that part back in.  But they do have solid folders on sale right now.  Might wanna check them out.