The Wilwood box will require a significant rework to the footbox panels. The advantage to the Wilwood box is the balance bar that allows for easy adjustments to the brake bias.The Mustang box will bolt right in and allow you to bend the pedals to suit your preference. If you're using Mustang derived brakes I'd stick with the Mustang box and master cylinder. It will also allow you to use vacuum assist if desired---don't forget that if you stay with manual brakes the brake pedal has to be modified to change the pivot point for proper leverage.

Jeff