Are they a custom set-up, single or double adjustable?
I haven't seen many off the shelf all aluminum set-ups.
I had to special order mine directly from my manufacturer.

Are these going on the ls1 rx7?
FYI, From testing different spring rates, the softer the spring rate in the rear, allows the car to plant and get better traction. This allowed me to run narrower rear tires, therefor less rotating mass and lower unsprung weight.