Keep the ramps under the wheels,but have the wheels about 2" above the ramps,that way if it does somehow manage to vibrate itself off the jackstands (highly unlikely) it won't fall very far. You do realize the wheels are still on the truck,so even if it did vibrate itself off the stands,while you're under it,unless you're one hell of a chunky guy,it shouldn't crush you,and with the truck in park and the front wheels chocked it's not going to run over you.
I don't really approve of your idea of doing it on a driveway facing downhill either,as that just compounds your worry,do it on a level spot,less chance of it falling off anything,and trying to run you over.A bit of common sense needs to be employed,and doing it on a parking pad that's not level isn't the best idea
With the way you're worrying about it,you might be better off letting a shop do it,that way if it falls off their hoist,you won't get hurt,lolI