Hi tomsa, Sorry, I lost track of this one for a while.
yYs, the diagram you show above is how you measure the arch. There is free arch, the measurement when the spring is unloaded, and installed arch, when it is in the car with all the weight on it.
I'm not surprised that Summit does not know the rate. Most people selling leaf springs do not know the actual rate of their springs. There answer is about what I suspected. They have a stock style spring that is built with less arch in it to achieve the lowering effect.
Unless you have done all the math to figure out your weight distribution, front suspension set up, and other related info to determine what rate you need for the car, it will certainly be easiest to get the stock XHD springs, get the car on the road, and then figure out where you need to go from there. Of course, that is not easily accomplished from your side of the pond any time you have to buy parts. If you want some help approximating what you may need, I'll give it a try, just let me know. As far as actual arch you need in a spring, again, we can approximate it.
The rear shackle is the same between E and B bodies from what I can tell. They should interchange.
