When I fitted new rear springs to my 4/4 I did it without removing the fuel tank, but I did remove the seats. I fitted the rear shackles first, and clamped a bar to the top of the spring that gave me loads of extra leverage on the spring. Then Mrs Hamwich assisted with muscle power whilst I lined up the front eye and pushed the bolt through.
What we expected to be a complete SoB turned out to be relatively straightforward.