Looking at the reply from ARk, via Adrian above, I can somewhat agree with their statement about the rear brace bars. I purchased a used Ark exhaust from a chap who had it fitted to a GT model....and I was fitting it to my Nismo. When it was all fitted up correctly the rear pipes were close to some of the rear bracing bars on the Nismo, but they did not touch once the system was all tightened up. Perhaps they are just saying this to add some protection should any rubbing occur?
I ran it on my car for over 1 year with no issues. The only comment I would make with fitting it to a Nismo Vs a GT is that the tail pipes do not sit flush with the bumper. They sit a little more underneath the car, which I can only assume is due to the Nismo rear bumper being bigger/deeper? But never bothered me.
In terms of the tailpipes not being lined up, or interfering with the diff on the Nismo - this is due to the mid pipes being fitted on the wrong side. Keyser and I fitted the Ark grip to my Nismo and on first fitment my car looked exactly like the photos from Payco. After some head scratching we swapped the 2 pipes over, tightened up the system, and boom! Perfect fitment (with the exception of the tail pipes sitting a little under the bumper as mentioned).