FWIIW, the Mustang has hydro-boost brakes for two main reasons. One, if modified with a big cam, the engine may not make enough vacuum to operate regular vacuum assist brakes.
Two. Booster to engine clearance. There just isn't much room for a large vacuum assist module. Remember that the 4.6 modular is a tall/wide engine. This puts a serious limit on how large the booster can be.
This situation is bad enough with the SOHC. The DOHC is much worse.