...and these are simple free rate calculations. A spring's installed roll rate can also vary based on mounting layouts and shackle angles. For your Charger, with its parallel mounting, it is a good starting point as the splayed layouts are only found in Late B, E, F, J, and M bodies.

To figure out the wheel rate of leaf springs, you need the center to center distance of the axle housing mounting pad points and center to center of the tread width. Divide pad width by tread width, multiply by 100, that's your wheel rate.

Your rear anti-sway bar also contributes to wheel rate as well.