The car for the stunt run that I use is the Hunter Cavalry.

So, the only stunt run that I've seen and played so far that could rack up millions of points is at the T junction at the end of the Southbay Expressway, which is located on the South Mountain Drive road.

Once started, start boosting and drift your way up. After passing the entry to the Quarry, make a U-turn, and start reversing just in case you won't be able to boost in time, to ensure the multiplier and points keep on going.

Then, boost all your way down and try to do a reverse flat spin + barrel roll on the SuperJump ramp so you'll end up on the road leading to the Southbay Expressway again.

By then, if you did this all correctly you should have at least a 6x - 7x multiplier with 15-20k points.

Then, head to the Southbay Expressway, and try to do some crazy stunts on the ramps in the middle of the expressway.

Try doing barrel rolls and reverse flat spins on the ramps on the expressway but try not to crash. If you're worried, just do the normal usual jumps.
Don't forget about the two billboards.

By right, you should have about 12x - 13x multiplier by now, or maybe more.

After that, immediately turn to your right and you'll come across a billboard, and then a Super Jump.
After that you'll land down and there's another billboard if you continue driving straight and jump off another ramp.

By then you should have around 15x - 16x multiplier.

Then, head back to the Southbay Expressway, and do the jumps if you missed any.

After that, immediately turn to the right and head for Casey Pass (where the dam is), and there's a billboard if you jump through the ramps on the right side. Try doing flat spins and knocking the billboard and you should have a near 20x multiplier by now.

Then after landing, head straight for the SuperJump at Casey Pass. Do some flat spins there and you should have a 20+ x multiplier by now.
You could do a normal flat spin on the first jump and when you E-brake and jump through the jump again you could do a reverse flat spin. It still counts.

It's better not to screw up anymore because around this time the timer should have ended and this will be your final run.

Then after that, drive straight to the Quarry, and land that awesome Super Jump with barrel rolls and reverse flat spins. You should have about ~ 30x multiplier by now.

Now, if you're familiar with the Quarry, you should be fine by now, just do all the jumps with flat spins and you should get the 40x multiplier.

Don't do the barrel roll on the ramp over the river, just to be safe. It's really difficult and you won't want to waste your accumulated points and the multiplier right ?