I have been trying to figure it out myself and have found very little helpful information. My "opinion" is that the PP front springs take a set, and in time drop the front end the perfect amount above the tires. However, the rear GT springs don't take a set and leave the rear end too high. My solution was to re-curve the stock rear springs myself so that I could determine just how much to lower the car. I'm not suggesting this as a long term fix, just as a way to figure out what spring to buy.
With 4000 miles on the re-curved set and several different reiterations I have figured out exactly the length of rear spring I need. I want the stock rate so the stock shocks will still be properly calibrated and front and rear will still be in sync.
Unfortunately I have found little published information on those specs from the aftermarket companies. All they supply is the "drop", and most talk about a firmer ride. I need the free length and the rate from each spring manufacturer, and plan on contacting each one directly.
Lastly I only want to buy the rears, the fronts are fine, I like the ride and handling.
Hopefully the money I saved from not buying a bunch of trial springs will pay for an adjustable panhard bar, a necessary component. https://www.allfordmustangs.com/forum...ml#post6136825
For comparison specs I am looking for are a free length of 12.5 inches (about 1.5" shorter than stock) and a rate of 140lb/inch. I know it must seem odd to take 1.5" off the rear and nothing off the front, but that is the dimensions needed to get a 1/2" tire gap difference rake with <40K miles on the front springs. At this point the rear end is offset about 5/16 toward the drivers side, hence the need for a panhard bar.