The AI in this race has extremely poor pit strategy so it's not particularly difficult to win. Just be ready with something around 700pp (and its La Sarthe, so it should have a high top speed), racing hard tires, rain tires, and you should be good. The rain can be annoyingly difficult to predict as the storms move through at
6,000mph (
seriously)* so your pit strategy is somewhat based on lucky timing - but the AI strategy is "wait until the track is undrivable on slicks,
do another lap, then pit for wets right as the rain stops" which is definitely worse.
The Porsche 962 can crush this race detuned to 700hp. I've also won it with a stock Ferrari Enzo (restrictor necessary), Mazda RX-Vision race car, Mercedes Mclaren SLR with full weight reduction (so it is easier on tires), Mercedes SLS Group 4 (not a great pick to be honest, its hard on tires and not that fast). The 962 has very good fuel economy, is easy to drive, and is way faster than the Group 4 cars in the race on the Mulsanne, even when power limited to fit within 700pp, so it's the one I would recommend. Road cars tend to chew through tires much faster, but it can be fun to try to win it with an unsuitable car. I think I could win this race with a completely stock SLR, but I haven't succeeded yet. She's a thirsty girl and the tires go away alarmingly fast.
*I seriously don't understand why there is even a level of zoom on the weather radar if the rain moves across so quickly. They might as well have left the crop at the immediate extents of the circuit and let the weather move at natural speeds - the effect would be exactly the same. The weather radar is basically useless as you can't
actually forecast with it, because it gives you at most
one minute of notice.