Two things...........
1) I believe Roush Stage 3, 360R, and 380R's all have 13'' rear brakes. You may want to contact them about their kit and maybe they can answer any questions you may have.
2) Cobra brakes aren't that great believe it or not. It seems a few of you think they are great. They are good for about one good high speed stop. After that, they aren't any good until they cool down again. And don't ask me how I know, lol.