so bascially the body just looks different?.. but i thought that the 94-98 looked different from the 99-04
No. Lots of differences. 99-04's use COPs instead of a distributer and wires. Interior for the most part is similar. Performance heads were used on the 99-04's. Do some reading and research. Lots of specs and info out there.
99' Rio Red Mustang GT:
Summit O/R H, MAC Flowpaths Turndowns, BBK Shorty Headers, Steeda UDP, K&N FIPK2 intake, Diablosport Delta chip, BBK 75mm throttle body, Trickflow upper plenum, Intake spacer, H&R SS springs, Cross drilled & slotted rotors, "Functional" hood scoop