My 12D RS Z has it's original EY "814" springs in it. I would assume with the OP's Z being a non RS it would have a different spring...?
In other cars I restore here I always use Coil Spring Specialties. They have done a fantastic job getting the rates and stances correct on everything I've bought springs for. A little more spendy than other places but I like the service and the detail in which they provide.
As an extreme example doing my fathers car, I threw a real curve ball at them, wanting a factory stance, but also a spring that would react and produce weight transfer similar to a moroso trick spring. Then told them it has a fiberglass hood, a heavier aftermarket block with aluminum heads, hidden headlights, and a couple other variables thrown in. They wanted all this information. They nailed the factory stance perfectly. Very happy with their services.
