Multiply the spring rate by the square of the motion ratio. Dimensions in the Mathis book have the spring moving a tiny bit more than 70% as much as the axle pivot, so half would be a good number to use to get wheel rate. So 100 - 125 and 175 for your two springs. In two wheel bump mode, anyway.
Couple of things to keep in mind.
Angularity of the spring away from perpendicular to the arm has a small effect (cosine^2 of the angular deviation from perpendicular).
The wheel rate in roll also depends on how far apart the axle pivots of the LCA's are relative to the rear track. And like all the other dimensional stuff as it affects wheel rate calculations, you have to square this ratio too before applying it to the two wheel bump mode wheel rate above.
Best info I could still find for the front is a M.R of 0.5 for OE type springs and 0.9 for C/O's. No info on whether any cosine function is included, though that would still be a rather small correction. You're probably within the range from 205 to 215 lb/in.
A forum community dedicated to Ford Mustang owners and enthusiasts. Come join the discussion about performance, production numbers, VIN decoders, modifications, troubleshooting, power adders, DIY tutorials, and more!