You can use the existing right side (passenger) rear pipe just fine. You only need to purchase the driver's side (left) GT pipe. They are the same 2.5" diameter. They fit nicely into backside of the 2.5" Magnaflow X-pipe. It is flared to accept 2.5" pipes on all 4 points.

When they installed mine, the pipes IN FRONT Of the X-pipe had to adapted---the X-pipe received the 2.5" pipe just fine, and then the two pipes to the manifolds are a bit smaller, and nested INSIDE the 2.5" pipes coming out of the front of the X-pipe.

Clear as mud?