[LH Lead-Foot]

I did not use "Variable Rate" springs on my 67, but with the photos showing the bottom spring perch position are different from side to side. I would loosen them to see what happens when you rotate one or both.

I don't believe the panhard bar is the issue. Just un-bolt one end, then push & wiggle the frame a little bit. I don't think you will see any difference.

I have installed hundreds of springs either for replacement or required due to other work, but they had pre-formed pockets to sit in. These truck don't. The mounts are where they end up when you tighten them down. Give it a try.