How to unlock offline!
You need to beat a Ferrari 330 P4 in a race while driving the Ford GT40 Mark II. I noticed that when playing a quick race with the GT40 you get 'matched up' with other A-class cars including the Ferrari you need to beat. In the only other solution available (at the moment) you need to play online in a community race but with my solution this can be unlocked offline as well.
Choose the following:
Main Menu > Free Play > Quick Race > Ford > GT40 Mk II > TopGear > Outer Loop
Make sure there is a Ferrari 330 P4 in the race by checking the Starting Grid in the pre-race menu. If there isn't one, just quit the race and select the same track again after the load-screen.
The race is three laps and you need to win which shouldn't be difficult if you are playing with settings (difficulty, assists etc) that match your playstyle and skill.
If you want to get the slipstreamer (full draft score) achievement as well try to stay behind the ferrari and when going into the long straight stay as close to the rear end of the ferrari as you can, the achievement (link below) should come quickly.
It is worth noting that the first time that I started the race I was matched up with a Ferrari 330 P4 and unlocked the achievement. However when replicating the circumstances for this achievement for the sake of this solution the car was not in the starting grid until I had restarted 8 times so it is very important to check the grid and restart if it is not there!
The achievement unlocks AFTER you press continue when the race is over.