Should I do a Cat delete on '75 Duster

Axe it. Unless your in California or some other communist state with draconian emissions laws.

Typical. Didn't even bother to read the details of the post and you don't know anything about California smog laws. The OP's car is exempt because of its age. His car never has to be smogged, or even inspected, ever again. As long as the title remains clear there's no reason anyone with any official capacity has to look at that car in person ever again in California.