I bought and ran that tensioner in my Magnum based 408. It seems to work well, but it is really just a chain guide. Once the side plates of the chain links cut through the nylon, the rollers roll down the nylon with little to no wear on the nylon block.

The best trick I've found to keep my timing chains tight is to soak them on their side in quality synthetic gear oil.

I stumbled across just how good of a break in lubricant that stuff is when I ran out of chainsaw bar & chain oil one time. I substituted Redline synthetic gear oil in my saw and the life of my chains more than quadrupled. It was nothing short of amazing.

Started soaking my timing chains in it, and it has virtually eliminated any chain stretch in half a dozen of my motors so far. The results are remarkable.I also use the highest quality timing sets available. .