i dont know...
it cant be just the exh, b/c even the 5.0 mustangs sound the same.
and i know of some pretty wildass 5 liter cars w/ headers, x pipes, you name it... they all have the similar sound that tells you it is a mustang, w/o having to see it... i think it may be the firing order thing, or at least something other than the exh. you can run no exh and it sound the same. and true duals on any other car (at least not a ford w/ a five oh) doesnt sound like it.... hmmm...