Over-Axle Conversion?

Spring-over-axle conversions are common on Jeeps, especially YJ Wranglers and they create a whole host of issues to overcome. Since you're talking about a Duster, you won't have to deal with steering linkage or front end geometry, but you will have major problems with axle wrap and drive line angle/breakage if you have decent power.

That being said let me tell you how I solved the problem on my 70 Duster.
I installed a short style add-a-leaf kit in my severely sagging springs. The kit I used is Explorer part # EXP13120 available from 4Wheel Parts (4wheelparts.com) for about $35. I did this about 4 years ago and was so happy with the result that even though I later upgraded the rest of the suspension (Magnum Force, Helwig, etc.), I have never felt the need to change out the springs. Yes the rear is now stiffer than stock, but it balances nicely with my larger torsion bars in front.
I've since done this to 2 other Dusters with similar results.