The first step is unlocking the 1973 Reliant Supervan III which is a barn find, found nearest the airport in the outback. After the car is unlocked, get in the car and load an upgrade from the storefront that is at B700. I used one by creator "Raceboy77".
Now, go to the
Airport Danger Sign, and get a good lead up to the ramp in order to shoot off with a lot of speed. You should be easily be able to attain 310+ft of the 300ft goal, as long as you land safely.
You may have to expand
The Outback festival hub in order to get this PR Stunt to appear.