Charley is right about checking the play in the distributor...but, I can tell you first hand that mileage isn't the best gauge. Our survivor 70 L78 4spd Nova had about 22,000 miles and there was excessive play in the chain, so it was replaced. Most folks couldn't tell it was ever apart. Our survivor L78 th400 car had a similar situation, about 30,000 miles...when I took the timing cover off the chain almost fell off into my hands.
That design was very poor given the performance aspect of the L78 motor...I wonder how much it would have cost GM to have used better parts to begin with...probably about a $5.00 back then
Just wipe off the grease. clean the gasket surface and put it back together...also, no need to remove the motor for a timing chain/gear swap...just pull the radiator and the other stuff and do it.
-wilma