Who ever told you they are all the same does not know their stuff. While it's true that 4R70W's are similar, there are differences. Remember that the 4R70W is a PCM controlled transmission. Expect some issues if you were to install an older AODE.
The other difference is the speed sensor. 99+ Mustangs use a VSS sensor. 98 and older use a OSS sensor. The two sensors are NOT compatible with each other. The sensor used must match the wiring harness and the PCM.
Note, some interchange with other cars is possible. However, expect some differences in configuration.
For example, the 4R70W from a 97 Crown Vic/Grand Marquis/Town car will fit but the DTR sensor is pointing the wrong way. So if you are willing to swap the DTR sensor, it will work.
Other transmission may fit as well with a tail shaft swap.
In general, for a "plug and play" swap, try to stay as close as possible to the original model year.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.