I also forgot to add, the wheel rate on the front end of an SN95 is 25% of the spring rate and the wheel rate on the rear is 50% of the spring rate.
I guess that's the third reason, and 'C' for the list above.
I know this is an old thread but just in case someone non-technically minded finds it in a search, I'd like to correct a misunderstanding.
The rear spring rate on a '79-'04 Mustang is so low compared with the fronts for two reasons.
A - the rear roll centre is ridiculously high (16") because...