69 Dart 7 1/4" leaf spring mod

Putting longer shackles on actually has the opposite effect on the spring. Yes, you are lowering its position in relation to the frame, but the added leverage of the longer shackle has the effect of de-arching the spring. Basically, if you add an inch of shackle, you'll only see a 1/4 inch rise in the car. If you want to boost the spring, go the opposite way. Drill a pair of holes in the original shackles midway between the factory ones and use that as the mounting point.