I believe the standard 500 and 126 front leaf spring is same configuration and interchangeable, but as the 500 weighs around 1168 lbs and the 126 weighs around 1279 lbs, there may be a slight difference in spring rate.

However, with only 111 lbs total kerb weight difference, and obviously with less affect on front, I would suspect that any difference in spring rate would be be so small as to be irrelevant.

Check with 500Mania or Middle Barton to be sure. Don't forget, if you wish to lower, you require a "reverse eye" spring.