Most of the road cars on that race go through a tank of gas in 2 laps tops when you floor them on FM1 (apart from 787 swapped RX7 obviously).
Try fine tuning it to PP700 and drive it on FM5 or FM6, short shift on half bar and you will be good. In my experience, even 2 laps on FM1 is ok because most of the time the rain comes at the end of lap 2, so then I pit for inters/wets, switch to FM6 and short shift through the rain. When it stops, back to the pits, racing hards and full throttle on FM1 until the end. If anything, it's fun and interesting.
Even if the rain doesn't come, these tuned road cars are so much quicker down the straight compared to the Gr4 cars that you can easily win the race even with 3 stops, you build up enormous advantage lap by lap.