Overload shocks on a Duster?

Jerame. There are two potential issues. The first has been covered and the second is potential for oversteer. It's only "potential" but if it happens it will only show up at the wrong place and the wrong time.

So be sure the car setup needs stiffer rear springs before going there. The overload or helper springs are for a vehicle that is always carrying extra weight or trailer. If the rear leafs are worn out or broken, these are not a good band-aid.

Panhard is much easier to do well than Watts. Definately should be considered when developing a car for very high speed events (such as road racing or the higher speed classes in Silver State type events). For vintage road racing, track days, etc a Chrysler leaf setup can be made to work. For street, the leafs will be good for the broadest range of conditions. A small rear sway bar can be added. But again, be careful of the possibility of high speed oversteer. Best to work with a company that has experience like Firm Feel or Hotchkis.

Stanley Springs (Harrisburg Pa) were all made here, and I assume still are. If you can't find a local retailer, these folks will ship:
Plymouth Leaf Springs
There's also a Dodge selection.

Another option, especially for autocross or track type use, is to adapt a circle track spring from Afco.