Been there done that. Pain in the arse aren't they.

There's just room to run a drill bit sideways throught the bolt each side of the leaf spring. Drill through the biggest you can get in there - undo the back of the sping, remove the U bolts from the axle, then wedge the back of the spring over to one side a bit to give yourself more clearance. Then finish off the bolt with a hacksaw blade - the bigger the hole you've drilled, the less sawing. New bolts and plenty of copperslip - job done.

Unless you're lucky, this also happens to the lower damper mounts (though usually only on 2.8s in my experience).