Hi. I suggest the following way forward;
1) Use a jack to have the rear end at desired ride height.
2) Measure the distance from rear-fender-wheel-arc to centre-of-wheel, and we have a starting point !!.
3) The rest of us should be able to chime in with our measurements and installed spring types.
4) If photos are used, we should state the outer diameter (even if this can be calculated if we have the tire data)
Now I don't know if there will be much difference between a Challenger and Barracuda,...but to be safe, we should state which E-body we're collected our data from ??
Also,...what are the experience with respect to sagging over time ?
I will NOT be surpriced if there will be atleast some spread in ride height even for similar spring-car combinations...