I to had massive problems with this race, it was extremely frustrating, and I consider it the hardest challenge of any GT game so far. I have golded every license test and every mission from the first GT. Every strategy listed on these boards I tried, and I purchased every single GR.3 car and tried them with no success. Best I could do was 2nd.
What ended up working for me was using the Toyota FT-1 with RS Front and RM rear to start with. Got up to 3rd just behind the leaders, and pitted lap 12. Changed to RS, filled up with 10.5 laps fuel, and came out of the pits in 3rd, just behind the 2 lead cars, which I was able to slipstream and pass with a couple of laps to go. From there it was relatively simple to block them from re-overtaking me, and victory was mine. As I says, this mission is hugely frustrating, however it's very rewarding when you finally beat it. Hope this helps somebody.
PS- I'm playing with DS4