You would have to cut you dp (its welded to the cat) and get connectors to attach back to the dp and cat back. Just buy a 3" dp with cat and save yourself the time, you would be better off with the 3" dp anyway.
The ATP dp will fit the Mopar exhasut, but the exhaust tubing diameter will go like this:
3 inch(dp)--->2.25 inch (reducer on end of ATP/beiginning of Mopar exhaust)---> 2.5 inch (rest of Mopar exhaust)
MAXX Fab sells a dp that eliminates he 2.25 inch bottle neck. All you do is cut it off with a hack saw, and install the dp. The dp is secured to the Mopar exhaust with a 3 inch wide stainless band clamp. This way the exhaust tubing diameter will go like this:
3 inch (dp)---> 2.5 inch (Mopar exhaust).....no bottle neck.
Maxx Fab uses Magnaflow hi-flow cats, same as ATP, and also make an interchangeable dp.