You can get an offroad pipe, which gets rid of your catalytic converters and of course that increases the volume. Of course, this is illegal in some places...
Also, you can do a mod where you cut holes in your flows and weld on threaded collars with screw caps on em. Unscrew the caps to open the holes and increase the volume. Here's how:
http://www.mustangworld.com/ourpics/fcar/cutouts.htm