The spring rate of a bone-stock '71 pushbutton was found to be 235 lbs/in. front & 355 lbs/in. rear. The rate of stock anti-roll bars was found to be 395 lbs/in- front & 150 lbs/in- rear. There are 17 full pgs of computerized suspension analysis on this pushbutton to be found on Ted Mitchell's website at
www.vaderacing.com.
FWIW, substituting black polyurethane for OEM rubber in suspension bushings does not change the car's appearance or spring rate of either the springs or bars, but does provide instant reaction to deflections and far more precision in steering, but no discernable increase in harshness. The trade-off is sometimes more suspension noise. After changing 100% of the suspension rubber for poly in our '72 (metal heim-joints on both ends of the Koni shock absorbers), I personally found NO change in road noise, and without elaborate greasing techniques being necessary.
I have written numerous articles for POCA on virtually all possible Pantera suspension mods over the last 22 yrs as applied to our highly modified street '72-L. If any are interested, all are available for download to POCA members at
www.POCA.com/documents. I'm working on an index of all 760 (current & counting) articles.