Well unfortunately I have no manuals to check but I thought that after 1996 the Mustangs all had 4.6 modular engines in them. I didn't think Ford used a returnless fuel system in the newer Mustangs so there should be a return fitting. If it is a returnless system rail it will not work with the old pump and pressure rail. I would think you would need to modify the rail with a pressure regulator and a return line. The returnless systems are regulated by the PCM which drives the pump.