Before you can hang body panels, you've got to have a straight body.
Is it worth the money to invest in a good frame alignment specialist? If you want proper door, fender, hood and deck lid fit - yes.
Time and time again, we see Mustangs at shows with poor body panel fit - much of it because restorers couldn't get a lot of it to fit, so they did the best they knew how and called it a day.
Good body panel fit begins before body work and paint. It is the very first thing you should do. Have a frame alignment specialist measure and straighten your Mustang's unit body before body work begins. If you have replaced floor pans, frame rails, torque boxes, and the like, have a specialist check your work.
When you have a nice, straight unit body, panel fitment becomes easy.