Should I do a Cat delete on '75 Duster

hey John, welcome aboard

one thing to keep in mind is that you are talking about a 45 year old car with a grocery getter engine

that engine was designed to run for ever, and the fact that you now have an engine that is almost 3 times your age proves that it does exactly what it was designed to do

however, you can not compare the slant 6 to a modern import 6 banger
in a lot of the cars you see today, specially the imports, the 6 is the performance engine

here, not so

even a great running slant 6 will not be a powerhouse
it will be fun, sure.
it will be adequate, of course

but it will not be a racecar


how, having said all that, following what 72blunblu suggested is a good start
and so is regular maintenance and tune ups, so do those first

oh, and post pictures