I have a freind who worked for Kelsey Hayes for a while in the ABS division. Him and the other engineers spend months dialing in those units so that they work flawlessly. The go up to the arctic cirle and spend days spinning them out on frozen lakes to get them to stop perfect every times.
If you just bolt in the brain from a different Mustang, it may work, but it may not work perfect all the time. Unless vehicle have the exact same handling characteristics, they won't be the same.
-Geoff