Funky clunk sound driving me absolutely insane!

[AirSpeed]

Been hearing a clunk sound on the passenger side of my 72 K1500. Jacked it up and found the ball joints were bad, just got done replacing them, went for a test drive and the clunk sound is still there! Had a friend with me and he can feel it on the floor board board on the passenger side. This happens mostly as I go around a corner. Checked all the tie rod ends, body mounts, recently installed new urethane bushings in the leaf springs, I can't find any source for this sound.
I did notice the front spring perch on drivers side moves a little when I turn the steering wheel, seems two of the rivets are worn, I can wiggle them with my fingers. I'm curious what I can do to fix that? Can I weld the rivets in? I doubt seriously it's the source of noise on the passenger side but I need to fix it anyway.
I am completely out of ideas as to the source of that damn clunk on the passenger side! Any ideas?