1964 Barracuda 904 swap

the physical part of putting the trans in is no big deal. it's all the peripheral stuff that's "gotchas"

you'll need a kickdown assembly for the trans (aftermarket cable is easy)
fluid lines for a cooler-- external if your radiator is not so equipped
shifter, aftermarket floor is easiest
driveshaft will need to be modified. 904 is shorter than a833, the output is dependent on what you have currently and what you're going to. basically flange mount or slip yoke.