Best thing I have ever found for helping timing chains not stretch is to soak them in Redline 85-90 wt synthetic gear oil for several days before installing them. I also tap on them while they soak in a ziplock bag on their side to get the air bubbles to float off and allow more oil into the rollers. This has virtually eliminated chain stretch on a half dozen motors I've had.

I stumbled across it when I ran out of chainsaw bar & chain oil one spring. My chainsaw chains last 5 or 6 times as long with it.