Does a Cadillac converter have anything to do with a 2001 mustang starting?
It seems to be getting common now for people to steal catalitic converters as they are stainless steel. They take them to recyclers to get money for them.
I have put straight through pipes on mine and removed the cats, but it still runs. Maybe they have done something under the car that is causing it not to start. Have a look around underneath and see what you can find. Hope they did not destroy too much.
These thieves have done a lot of damage to a lot of cars locally here !