Welcome!
Based on how high the car is sitting in the back, I would have to say I doubt those are stock rear leaf springs. Maybe XHD's or super stocks, but not stock. Also, if the springs are new, they will settle in as the car is driven.
If the springs aren't new, the best way to lower the back of the car would be different rear springs. Hotchkis makes a rear leaf that lowers the car about ~1" in the back from stock (your car is sitting higher than stock!). ESPO makes springs to lower the car as well, the amount of drop can be specified.