1. did you look at my post, “Rear ventilated 300mm disk”, 2. in your photo it looks like one of the pads is in backwards? Also are those AMG calipers with a sensor? The SRT and NA do not have rear wear sensors, the #1 photo on my rear 300mm upgrade for a NA shows the caliper you need. If you got the SLK 3.2 AMG rear calipers I do not know if they will go on the Crossfires.
what year and type car are you doing this on? Even tight pads will not score a disk like that.