I got the mopar catback (corsa) for the 6.1 when we bought our r/t. I had to have the cat pipes expanded just a little bit so the catback would fit because srt exhaust pipes are bigger. Other than that they fit perfectly.
That tool may or may not work, Idk. Easier to just pay a good muffler shop with a hydraulic pipe machine. I think I paid 100 bucks, and didn't have to roll around under the car trying to install the catback. Well worth it to me.