I agree you'll probably have more bang for your buck if you spend (some of) the money on your stock suspension.
The engineers at the factory were experts at making suspensions that drove and handled nice. Just read all the magazine roadtests of the era, comparing them to GM's and Ford's offerings.
Our cars may have out handled a Ford or Chevy at the time but a modern minivan will out handle it today. So upgrading is the thing to do. That being said I went with the Hotchkis TVS system as it looked like balanced system that will work together. and lots of bracing and welding of seams.
If I upgrade at a later date I would look at the Magnum Force rear suspension but it is weld in. It will also give you more tire clearance.
