Rear springs/shackles question

[maxreps]

Bought new 4 leaf rear springs for my 1/2 ton '76 long bed step(has a flip kit). Two reasons - the ones on it were prob original and they seemed to bottom out too easliy when carrying mulch, etc. the 2nd reason was that unloaded the rear was very stiff, so much that I thought it was hitting the factory bumpstops(had them removed and it still was harsh, so I know that wasn't the issue). My question is that when I'm tightening nuts on the shackle bolts, is the shackle design meant to have the nuts tightened to "pinch" the shackle/spring together? Or are both ends of the shackle(spring end & frame mount end) meant to pivot freely as if they were a hinge joint.
I saw that the bushings in the new spring's ends have "teeth" on them so I assumed the nuts were supposed to be tightened down enough to "pinch" the shackle to the leaf. Making those teeth grab.. But after driving it, the ride is still pretty harsh so then I wondered if the nuts should be loosened to allow more free movement.
BTW, the ride height went up .75" drivers & 1" psngr side, both sides are equal now too. Pleased with those 2 things but am still looking to ease the stiffness up a bit.