Anyone have that spec available. I did a search on it, but I'm debating whether I'm going to M2 550/450 setup with my cutout fenders and fender flare setup.

 

The general consensus is approximately:

Fronts: 280 lbs/in
Rear: 200 lbs/in

Note this is for all models as springs were universal between the models.

Without changing any other aspect, I would think it would be best to maintain a 1.4 stiffness ratio between the front and rear. Of course if you run staggered wheels, a larger front sway bar, etc, you might like a stiffer rear end. Max Cooper has gone through several sets of different springs, do a search on this forum and his website for more info.