>"In the days of the Slant-6, cars never got to 300k miles unless they had ridiculously meticulous maintenance"
As the former owner of mid 1960s Slant 6 purchased with ~ 220K miles on it that I personally put another 120K before the block cracked I am telling you that you claim of "never" is just plain wrong.
Aside from periodic oil changes I wouldn't describe my maintenance regime as ridiculously meticulous at all.
There are many vintage Duster/Dart/Valiant owners with cars that have 200K and greater miles on them. Here are some links with attestations that corroborate this fact: