The Indy Speedway Endurance in McLaren Generation (I forget the exact stage) spawn many more bots than usual past the 25mi mark. With a 29/30 and front damaged P1 GTR I never ran out of bots to pass. Ended up stopping at 100mi. (40 laps) but 500 mi. seems easily doable as I saw no slowdown in bot frequency. I'm guessing the sweet upgrade spot is max (or near max) grip and top speed of at least ~230mph on the straights. Even the fastest bots slow down on the corners, allowing you to pass them easily due to the tremendous grip.
Payout is less per min. than the Le Man 10-lapper's but if you have the time (and bonuses) this can easily pay out millions. No agent, no daily bonus payout for 40 laps is R$ 142K/Fame 24K.
v4.1.5 changed the bot behaviors and it's now much harder to keep the train going indefinitely (see post #190). However, from my observations in #192 I believe it is still possible as long as you can keep your car damage-free.
Dead as of v4.7.2.
It's back in 4.7.3.