The chains may not break that often, but every year we see a few unlucky friends here that have catastrophies with something breaking in there, most likely a slide. I say most likely because there is so little left to work with after that happens.
I say just change those parts now, much cheaper than the alternative. Besides, after changing the timing chain, with over 330,000 on my 78, it lost that annoying idle miss.
Can't say the same for my 81, but that was the problem with the older beast. BTW, it had 8 deg of stretch.