Your mileage is low, I also suspect a stretched timing chain. Those stock chains are good for maybe 50,000 miles...I have put a chain and gears in every /6 I owned. I have seen them stretched at much lower mileage as well.

As far as gears why not 2.94 instead of 3.23?. I have tried both with the /6 and I always found 2.94's worked best.

One /6er was a 69 coronet deluxe ferder [4dr] with no options...3 on the tree. I got 24 mpg on the interstate running 70's with 2.94's. Car had 3.23 when I bought it & got 19. The proverbial lol car, it was Grandma Websters car