So visit your local Pep Boys and get a matched pair of "Freeway Blasters". 132 decebles. About $15.00 each. They will both sound nice and be loud enough to get you noticed. It won't break the bank either. They will sound a lot like the Mustang horn you are talking about, but louder.
The SVT dual exhaust is a bolt up onto your car with the exception of needing to fabricate a right side rear hanger. It will hang below your rear bumper. Check the CEG classifieds. You can nearly always find one available there.
