I got the catback from MRT & mine did not fit the best but what I have a problem with is the clamps mine leaks around them we put them on all kinds of ways but they still leak.
My bud told me to get Borla Stainless Steel Band Clamps & they should fix that
My magnapacks on the GT and the Magnaflows on the cobra both sealed fine with the provided clamps. Maybe your catback pipes need to be adjusted better.
I just ordered a magnaflow catback and it will be on by end of this coming week, Afew of my friend using the same catback said the best thing to do is get it where you want it , then weld the mufflers to the flow tubes, and get exhaust band clamps(from summit or jegs)and strap those to your muffler and tail pipesthat sould solve your leak problem