Call the company and ask them. They are the ones who test the items.
Yes streaching the spring will cause it to have more return pressure, however they also know how far the springs can streach while still returing to it's original shape, and how long the spring must become during a maximum right or left turn. If the springs start out longer, and then in a maximum turn are streached say 2" farther, it might cause distorsion in the spring, leaving them perminately longer than origianl.
Just a thought.
Fred.
PS: Are you having problems with wander? Or the steering not returning to center? Or tire wear un-evenly?
Checking the tire lean in and lean out can tell you how well it can track down the road. Some alignment shops "Claim" there is no way to adjust the lean of a tire on a straight axle chassis, such as my F-53 Super Duty frame (97). However a good truck shop, with 20 ton jacks can make adjustments, and did mine when it was 3 months old, with 951 miles on it. Never had to make an adjustment in the past 17 years after that, and the tire wear is really even.
If you have this problem, then you need to find a truck alignment shop that is over 40 years old, and knows how to deal with straight axles. The place I went to has been in business since 1948.
Fred.