The first step to this achievement is completing 4 "Showdowns" which are the events at the very end of each category of races in Career mode. You can choose any 4 categories you want, but I recommend doing the GT category as one of them (more on that later). Once you have completed 4 of them, the GRID World Series at the top will unlock, which also gives you this achievement:
The Final Stretch achievement in GRID worth 244 pointsGain entry to the GRID World Series Next you must complete all of these races in the GRID World Series including the "Showdown: Ravenwest" event at the end, which unlocks this achievement:
Now exit career mode and go to Player Profile and select Team and Nathan McKane will be unlocked to buy as your teammate for $150,000. Now all you have to do is win a race using the Ravenwest livery. Go back to Career mode and near the end of the GT line is the Sunoco Prototype Super Sprint. If you already completed this before, you'll already have the car and before you start the race press
to select a livery and under the Stock category select the Ravenwest Motorsport livery. Continue on and win the race and the achievement is yours. haugse23 points out that the Cadillac in this class has the livery available but the Acura does not.
You may also be able to win a race in Free Play using the Ravenwest livery but I didn't try that before unlocking the achievement. Also if you're trying to unlock all the achievements you will have to complete all the GT events anyway and buy the car for it.