Chains do stretch. That is why they recommend replacing them when there is over 1/2" of play.
Tensioners - I believe that they came on some of the truck engines sometime in the late 80's or early 90's. HemiEd would be a better guy to ask that question. He had a 91 360 that he sold us that had a tensioner on it. I'm not sure if it originally came on our engine or he put it on from another one.
