What are you trying to do?
Mopar leaf springs should be nearly flat when static for most uses.
For drag race launch, there should be some arch. Longer shackels can be used to get a little more swing (unfortunately also allows more sideway but not an issue for straight runs) as long as they angle back from the frame to the rear spring eye.
If more anti-squat is needed, then raise the front spring eye. That's when a longer shackle is probably most needed for a drag car. For road race the lower hieght reduces the roll center, and roundyround/road racing/rally/street can't take advantage of unclamping the rear half the spring pack - nor should they need to.
Correct the pinion angle with shims. IIRC Speedway has 'em 2.5" wide.