The rearend and the disc brakes won't fit. You could pull the motor and tranny and dump them into the classic. Then sell the fox body. The 5.0 is a roller block and since it is an 89 it is Mass air car which are super easy to add performance upgrades. A couple of the main things you will need are a conversion harness, conversion tranny crossmember, electric fuel pump in conjunction with fuel return line and a different radiator. Then you will have a "New Classic Mustang" with crisp throttle response and better mileage.