1964 Barracuda 904 swap

Hello I have a 1964 Barracuda with a mild built 1970 318 and a 4 speed thinking about switching it to a automatic I have a built 904 and was wondering how much of a hassle it is

It's easier if you have the correct early 904 but not that much more complicated if it's a later version. Tell us more about about the trans you have.