I forgot this was for a rally car so I'm not sure my advice would suit. I was thinking more along the lines for a track car.
Flighter, All they do is reverse the way the leafs are made so they essentially lower the car by the height of the eye is that makes sense. They then add another leaf to the front of the spring pack to help reduce the tendancy to bind up under acceleration thus helping to prevent tramp. You also need high quality shocks to help out too.
I'll see if i can find some pics of reversed eye leaf springs. Wozzah on here can sort you out if you need some I think.
Matt