Do they have separate canisters for each side (I'm assuming they're running dual exhaust)? F-bodies and a lot of other V-engine cars have those horizontal crossover mufflers that just have two pipes sticking out of them at 90 degree angles facing back. If the Mustang has two straight, 14-18" by ~10 inch by ~5 inch oval mufflers, that could be a good place to start looking.