Seen at least a hundred post over the years, "I bought 1" lowering springs off the (fill in name) website and they raised/lowered/didn't change (pick one) the stance..." If you have original springs they sure don't sit the way they did from the factory and a 1" stock lowering may raise or lower or do nothing... If you have non stock tires (few run OEM skinny 14" rim, 5" wide high profile bais ply tires these days) it makes a difference...
The notes say Shelby drop, so that lowers it about 1/2 from stock already, so buy a "1" lowering and you drop 1 1/2" from stock. But with all the after market items noted, no telling how it sits now compared to factory.
If you want to roll the dice, buy a " 1" lowering" Scott Drake set. If you want a specific stance, measure the car and call King Springs. Just trying to save you a headache as many have dealt with when picking off a chart....