The main difference from the V6 to the V8 axle back should be the sizing of the exhaust tubing. As far as Magnaflow goes for example, the V6 kit uses 2 1/2" tubing while the V8 kit uses 3" tubing. This may or may not be interchangable but frankly it's your dollars to spend and find out. Worst case a little welding would be required and at that point are you even saving money? Muffler for Muffler the car will not sound like a V8 simply because you installed a muffler that was once on one