When I tried to get confirmation of which KYB struts where stiffer some time back, there tech dept said gen 3 struts were stiffer than gen4, contrary to what I had seen posted.
It's like that KYB shocks are stiffer than Motorcraft which would work better with stiffer cargo springs.
Stiffer springs need stiffer shocks/struts. It's a matter of physics. This is why I called the KYB tech dept months ago.