You asked if it is legal.
Any alteration of your stock factory exhaust is in violation of Federal Law. If an exhaust component wears out, breaks or is damaged, it must be replaced with the OEM part, or it's equivalant, in order to be in strict compliance with Federal Statute.
'96 T-Bird LX 4.6L Alive & well & living in S. Florida
N/A: Renegade NPI heads/Comp Cams; OEM intake manifold; Bolt-ons
245 rwhp; 13.713 @ 99.35 (Commerce, GA - November 2003)
BLOWN: Renegade NPI heads; Renegade shortblock; Intercooled T-Trim; some other stuff
Pump Gas Numbers - 547 rwhp / 525 ft.lbs; Best Time - 11.388 @ 118.68 (Commerce, GA - November 2007)