This is an old AD that has been revised;
The original issue of this Airworthiness Directive became effective on 31 March 1975.
The only new aspect I can see is that they allow continued operations beyond the spar/airframe retirement limit if a maintenance/inspection program is approved and used. Sounds like the 1970s Navajos may just keep going forever.
I have heard that the modification and inspection process for the initial wing spar retirement is not much of a drama. However once the second spar life and therefore the airframe life limit kicks in its a different matter. This was always around 26000 hours on a Chieftain.