It would help if you specified what year and model 2 door the exhaust is from, and what year and model 4 door you intend to put it on.
Answering with the assumption that they are both LS, you will need to add 'about' a 4.25" extension between the b-pipe, and the muffler. It must be done at that location. If you add an extension between the cat and b-pipe, the hangers will not line up.