The bodies are as flexible as a Chinese gymnast. If the car does not have subframe connectors then I would pay very close attention to how straight the body is. Look at the body lines and gaps (around doors, fenders, etc) for proper spacing and fitment. Have a good look under the car and pay particular focus to the torque boxes (where your control arms hook up to the body). These come spot welded and are a weak point, and it doesn't take much to rip them out! Also, try to find one that already has the basics done, like exhaust work and gears. You'll be saving a lot of money if you do. Good luck and welcome to the Mustang world!