The first two "Superman" ( Christopher Reeve) movies have several various 71-73 Mustangs scattered throughout both films.
Both movies were essentially filmed at the same time, and all in the UK at Pinewood Studios. ( also called "The Bond Studios" for a long time).
They imported as much iconic american stuff as they could for set dressing to really try and sell the idea that it was in America.
What could be any more of an iconic American image than a Mustang?