I think verticaly your fine with the dead weight, but I would add a second top plate to the ramps. The big concern that I saw is putting a lot of weight and pressure on the ramps that's not static(dead weight). I would shear nail the heck out of the sides, and possibly use mdo instead osb. There would be a lot of horizontal force being applied to walls driving up n down them, and the weight dist of the car sitting static, so I would be more worried about them folding, instead of kicking out.