Sorry, I'm no help.. I did see a chart of different spring rates at some point, but no idea where it is/was...
I am curious as to this too... I need to pick up some stock rear 56"s for my setup, and saw the stock replacement, cheapies over at JC Whitney where listed a 8 leafers.. I'm curious as to the different 56's available, spring rates, etc..
Maybe I can hunt down some info today during wiring breaks....