The transmission from a Modular V8 will NOT bolt up to a V6. The see the difference, count the number of bolts connecting the starter. The V6 has two. The V8 has 3.
Further, the swap is not a good idea because of differences in the speed sensor. The 98 and older Mustangs use a VSS sensor. The 99+ uses an OSS sensor. The different sensors are not compatible with each other. Further, the sensor type needs to match the wiring harness and PCM.
You may think that all 4R70W are the same because they have the same name, this is not true. It matters the model year and engine that the transmission came from.
If anyone is considering a transmission swap it's best to match the engine type and stick as close as possible to the original model year IF your goal is a "plug and play" swap.