My father and I just converted his 3-speed small-block challenger to a 4-speed this summer. It is a bolt in operation, you will of course need a new shifter mechanism and the only hiccup we had was the bolt holes on the bellhousing....they were there, but since they sat outside the 3-speed mounting holes they were full of grime so make sure you clean the threads up. I also know that it is a bolt in operation for big blocks because my 383 Challenger was originally a 3-speed and is now a 4-speed.
