It's actually very easy to do with the G65 with the right upgrades/tune and a good path. Upgrade it to the max. Drop the final drive a few notches. You're going to want to be able to hit 150-160 MPH in 5th gear to be safe. With rally suspension, you're going to want to stiffen it up otherwise the car will bog the suspension on landing. I raised the suspension stiffness about 10 notches away from maximum (both front and rear). Drop your tire pressure to about 28PSI. You can adjust torque bias to 60-65% rear if you want.
Do it in single player, and turn rewind on. Here's the path which gave me 559 feet. Took only a couple minutes. You have to hit the ramp at about 140MPH, if your speedometer shows 131-132MPH during the slowmo, you should make the 550ft minimum. If it doesn't, rewind until you do.
Set a waypoint on the stunt so it shows you precisely where it is. Then follow one of the dirt roads leading towards it. Just as the dirt road begins going left and uphill, veer offroad into a couple trees there. You're going to want to aim between the trees (or not). At the crest of the hill, you will get some airtime, but you should be heading straight for the jump. If not, rewind just before the uphill portion and use the trees to adjust your course.
Here's those trees just before you go offroad.