You will need to change the motor mounts, throttle linkage, and transmission bell housing. (It probably has a c-4 with a removeable bell housing) There will be some small things like fuel line, radiator hoses, etc that will need to be changed or modified also.
It would be a good idea to install a bigger radiator, higher geared rear axle, and 5 lug hubs. A front disc brake swap from a ford granada would be a good idea too.
The only thing that I remember having to be done to prepare for this swap is the motor mounts, but I would not be surprised if the radiator also had to be changed (along with the hoses)
It is a fairly straight forward swap, I would recommend going with a good MSD system, and make it an overdrive tranny (for fuel mileage)
I am only a girl, but I know my sh*t when it comes to "MUSTANGS"! I use to have a 1966 pony car, with a 289 V-8.
The 6 cyl, tranny, and drive line will have to be replaced.
To put a 289 engine in, the tranny will have to be changed over as the bolt pattern on tranny housing is bigger.
After installing a 289 and tranny, the drive line from the 6 cyl, will differ in length.
The differential can remain in your mustang.
Engine mounts will definataly have to be changed.
Radiator will not have to be changed.
Boot and shifter will have to be changed also.
There is really nothing to it, when you change over, always remember to pace yourself so you do not over exert yourself.
If you do the work yourself, use theory and ask yourself questions. Such as removing a hypoid cover with 5 bolts. After removing bolts and cover, do not just yank the cover off, as it may be spring loaded.
By yanking cover 5 or 6 springs may pop out and you may only see 2 shoot out you may assemble unit being short the springs you did not see pop out.
Removing any parts should be done carefully and slowly, asking yourself, it came off this way, would it go back on correctly if installed 180 degrees off.
You may see that the bolt patterns do not align, and cover must be installed the way it came off.
An easy way to remember how things are installed is by putting a punch mark on cover and case that it comes off of so they line up dot to dot. "GOODLUCK"! Oh yeah, going from a 6 cyl, to a 289 V-8 is a very good change over.
As the 289 has been a well proven engine from day one! "STANGS" are bad to the bone muscle cars especially the original mustangs the first came out of the factory line.
Transmission would be a good idea, but I guess it depends on what you are doing with it.
I would also suggest investing in some extra reinforcement (shock tower to tower bracing, etc) to handle the extra weight - not a bad Idea to bolt or weld some extra reinforcment in quite a few places on old mustangs - the metal gets fatigued, and they don't really have a frame.
Dude all you need is Money $$$$ and a good shop.. but why the hell are you settleing for a small *** 289? sheesh thats a kiddy motor even for ford.. .. put a 427 cobra jet in that bad boy if ya got any balls (well and the money) a 66 Mustang? you could make beack the money on sleeper races on the weekend.. no one would expect it LOL.. yeah you';ll need to change the rad and the tranny too.. Auto tranny would be good they shift faster in1'4 mile drag ntah manuals do for street races if you have em done right.. but that 289 isnt gonna do it.. that wont even beat a 318 dodge 4 door dart granmamobile any ehere anytime