When I was growing up in Nebraska, and the Highway Patrol was using the Fox body 5.0 Mustangs and Camaros, I was told that they had built motors in some of the cars, and when they got a replacement car in, they woudl rebuild the old engine and swap it with the new one. When the car went up for auction, the built motor came out and the stock one went back in. Not sure if it's true or not though.