The simple fix is to buy some relocation cones and bolt them on. Alternatively you could make some for the bottom of the spring by replacing the retainer plate with a 3 sided pyramid of angle iron or bar and fixing the top of the spring by making retainer bar.
Relocation cones are cheap enough, these are good... (not sure why they show 4 in the picture as you only need 2, obviously).
http://www.gwynlewis4x4.co.uk/page23.html
Or you can use these for "extreme" articulation...
http://www.gwynlewis4x4.co.uk/page68.html
All these methods allow the sprint to dislocate, giving you maximum travel, but make sure that the spring relocates correctly afterwards. The extreme one is only need if your spring is likely to dislocate past the end of the cone... don't think you'll have that problem.