TRACKBAR, anyone here run one?

Anyway you slice it a panhard bar describes a radius of a circle, thus forcing the leafs to slightly shift from side to side, "something they do not want to do."

No argument there, but at 0.040" total forced lateral displacement in the first 2.5" of up travel the bushings both won't notice it and left to their own devices will allow 2X-3X that. So we're not talking about bending the springs sideways. We're talking about a consistent lateral location of the rear axle that the springs by themselves can not do.

A "perfectly" designed Watts bar would not do this, but I'm not even sure this is exactly possible.

For a set amount of suspension travel it is possible. What complicates things is trying to fit the perfect design under the car.

It would be interesting to see what "the Petty boys" actually did back in those days. Seems to me there's some photos of his 70 upside down............

Agreed, it would be, but keep in mind that what ever they did had to attempt to meet the rules. So they quite probably didn't do everything that they wanted to do.
And just because they didn't do it doesn't mean that we can't or shouldn't. If you have rules to comply with, then you're limited by those. If not then something as simple and common as a panhard bar isn't rocket surgery and should be considered as one possible location solution.

FWIW only drag racing 4 links need lateral location because the links are all parallel. With single or double converging links no lateral locator is needed. Take a look at the GM mid and full-sizes from the mid 60's up. They're of the double converging design type. I can tell you from a misguided friend's experience that putting a panhard bar on them locks out the suspension.