According to eibach's website a 500 spring will work. A stock 250 is 4.0kg and a 500 is 5.0kg.
If that is not enough then a banshee rear spring 97 and up is rated at 5.5 or 5.4 depending on years.
Older banshee springs will physically fit but the spring rate will be too high at 6.8 kg.
So john can sell you one of his 8 or 9 zilla springs he has hoarded.