It will be much easier to fix or replace the 3.8. To convert to a 5.0 you will need a donor T-Bird of the same year or very close to it, to get the motor, accesories, flex plate, wiring harness, radiator, fan, motor mounts, computer, tach, ignition module ect..ect...
To make matters worse, when your all done it will be slower than the SC motor was. Don't do the 5.0 swap unless you going to use a power adder.
David