Putting ramps on a slope is dangerous, not an engineer myself, but it probably puts forces on the ramp that it is not designed to withstand. The literature with the ramp probably advises against using them on anything other than a firm level surface.
About twenty years ago I did this, my friend stopped by and I got out from under the car to talk to him and very shortly thereafter the ramps collapsed.
As others mentioned earlier, by pulling the pan you should get a good drain at almost any angle.
Do it the safe way and good luck with the job.