what can I do to prevent swaying and rocking?

have converted an LDV 2003 luton van 3.5 tone twin wheel to a horse box. Extremely light weight materials to keep weight down. Van carried 1,000 kilos of horse feed to spain no probs. BUT when loading my horse the van rocks and rols as the horse moves inside to load.
SO will not drive it with being like this when van is stable. What is there to add to the springs to make it unswayable. ITS not over weight as horse weighs only 500 kilos. and have weighed the entire van to check. Single springs are what it has currently. IM in Spain so parts unavailable no doubt here. WILL pay whatever it takes to remedy the problem. bring parts with you Ile pay air fair. cant use van till its unswayable. Drives with a load fine but will not chance driving it with a horse onboard in case he moves and van begins to sway about.