I am running 8 piston front Brembos and 4 piston rears off of an SL55 Mercedes on my car, and I just looked up the OE master cylinder size and found a late model Corvette one was nearly the same and just used that..was much easier in my case to use that one then try and adapt the Benz one. With a vacuum booster as well, and happy with the combo as well.

Using the Mustang master (and booster even) makes plenty of sense as said...since it is already going to be matched pretty much to the combo you are using.