In simplest terms, the rear tires breaking loose; especially at higher speeds and/or slick down hills.
When that happens, its only a question of how much damage will be done.
Borgeson, if you can get it all worked out, is fine.
Probably best to go with Peter Bergman on that since he's dealt with the issues.
Don't need the following:
-PST Adjustable strut rods
-PST Pivot shaft
-PST Tubular Upper control arms
-PST Super poly front end kit
For street use I'd use polyurethane on the sway bar end links.
Maybe also the sway bar bushings - in which case get the 'polygraphite' or greasable mounts.
And if you are willing to do some fitting and measuring, the strut rod bushings.
With a B-body, I would like to think the aftermarket strut bushings are the correct dimensions, but assume they are not. See if AndyF or another trustworthy source has done the work on this. Hotchkis set up and run a B-body on tracks - so that's a good sign their stuff has been worked out. But your tires and intended use calls for a softer setup. There qalso may be some differences in '66 vs. next generation (for example in weight, or weight distribution) so keep that in mind.