Looks Like I should be able to get it done at a mates metal fab shop for cost of materials.
I hadn't been thinking of the effect of the weight as having loaded it up for camping B4 it did not sag much nut then I didn't have 80+kg on the back. i think i'll build it load it up and see what happens. Might just put some coil spacers in to lift it back up for the duration of the trip. being the rear it would be real easy to take the spacers out as the load lightens. Or would that just trash the coils before there time?