I agree with "Sugar Bear." I would ABSOLUTELY not put ramps on a slope. The issue is more one of geometry than strength of the ramps. I'm not an engineer, but seems to me that with the ramps on a slope, the force applied to the ramps would have a tendency to make the ramps tip. Also, I would not be confident that the wheel chocks + the little stops at the ends of the ramps would keep the car from going down hill and off the ramps . . .
PLEASE don't do this!