The biggest modificatiobs that are seen on mustangs are the wheels and tires, these are mostly modified to be more visual pleasing and for a performance uprade. The lowering of springs or coilvers to make the musang overall lower. The exhaust to make a difference in the sound. Forced induction which you would use either a turbocharge or subercharge to fource more air into the engine. There isnt a number one body shop to take your car but the most globally famous is West Coast Customs which is located in Burbnk and is knows for celbrity and extreme body build shop.