The Mountain Top Danger Sign is on the west (left) side of the map. Roughly center on north and south. It's between the Blimp logo-ed showcase and the small lake with an island in the middle. The sign is partially hidden by the Mountain Scramble Drift Zone. You approach from the west (left).
700 feet is needed for 3 stars. The speed needed seems to be around 130 mph (209 kmh). This is give or take about 3-5 mph either way depending on some Danger Sign variables. If you break the stuff at the top in the right way (or avoid it all together) you can get some extra distance. If you turn into the ramp correctly as well you can spin the car so it lands sideways to gain a few extra feet. Finally if you make the right approach you can add some extra distance. I noticed hitting the ramp on the right side and turning left gives you more distance, but you have to be careful as there are unbreakable trees on the left (and right) below that will cause you to fail. Rewind is your friend. If you're getting within 25 feet or so try rewinding a little bit, if you're closer to 650, rewind more.
As for vehicles, I got 780 feet with the 2017 Nissan GT-R maxed out. I tried the Nissan Titan truck after from the Blizzard Expansion with a maxed out tune and it eventually broke 700 feet. It's acceleration isn't as great but it handles the dirt better than most of the Nissans. It should be doable in a maxed out 1973 Nissan Skyline that's a barn find, but I found I was flipping it too much and gave up.