That is total BS from your body shop manager. It is a well known fact that there are rear bumper fit issues on all models. A GT350 is built as a GT350, not a Mustang turned into a GT350. There is very little in common with the major components between the 2 models. The entire drive train, body from the A pillar forward, interior goodies, suspension and trim are different. Think about it . . . the factory doesn't turn a Mustang into a GT350, that's ridiculous!! And these cars are fully assembled at Flat Rock, nothing is done to them after they leave except get delivered.