Plymouth duster rear suspension questions

Bryan, I would advise you to use @DoctorDiff 's 1/2" spring relocation.
It is pointless to do the "in the frame rail" relocation if you don't have the car mini-tubbed. Furthermore, if you are going that route it is good for fitting a slick or big tire to go drag racing. For handling, it moves the spring too far inwar for handling dynamics and will straighten out your leaf springs. Mopar built these cars so that the leafs "toe in" at the front hanger. This does a lot for handling.
Moving the spring perch and leafs in by 1/2" will put it under the inside of the factory wheel tub and keep the "toe in". If you aren't mini-tubbed why would you need the leafs any further inboard than the wheel tub?