Dom, have a look at this. I did look into a combo Coil over leaf spring setup for my 75 series troopy, and went and drove the demo vehicle set-up, very impressive, but what turned me away from it was the 6 leaf spring pack was reduced to 2 leaf springs and the coil was mounted over the axle. I was concerned about stress on the long leaves. I kid you not, this demo vehicle drove straight over gutters and roundabouts as if they were not there, but the question in my mind was for how long before the leaves broke.
Anyway, if you want to strengthen the leaf spring set, whack a couple of extra leaves in and thats it. Its a smash about trailer, not a rolls royce, so dont waste time trying to get the thing to ride like a helicopter. Attached is a sketch, you should be able to cobble something together. I think you are local to me, so if you want a hand on a weekend, PM me and we can get this sorted. Your springs just need to be the same width as what you have. Length doesn't matter as we can cut them dowm. The centre bolt is usually a HT unit, but really a gr 4.6 would do and you can hand grind the head as well.
Attachment 74355