At the same time, Carroll Shelby wanted to make a Mustang which was not only faster than stock, but also actually usable on the road. The GT350, though street legal, was never really at home anywhere but a racetrack. 1967 also saw Ford offering the 390cu-in V8 for the Mustang GT, and as this used the same block as the 428 Police Interceptor engine, Shelby had an easy time swapping the smaller plant out for the bigger one. It officially produced 355 hp, except it really didn't. As we mentioned earlier in this series with the Trans Am, this was an era when automakers could get away with underrating engine output, and the 428 is said to actually have produced at least 410 hp.