From my experience with HD suspension, you need at least 2-300kgs of extra load to make them ride soft. 500kgs is about perfect.
I had a kaymar dual wheel bar and bumper, plus a load of junk in the back of my troopy and it did not soften the suspension all that much.
In Australia , the 75/79 series utes are workhorses and they spend their entire life with half a ton of tools etc in the back, this what HD is made for.
So maybe you do need to take a leaf out.
And you need top class shock absorbers to go with HD because when the suspension bounces, cheapies will not dampen them. It just wears them out quick.
I also recommend playing around with tyre pressures.