as you have posted on a couple of sites and received a considerable amount of good information you may already have everything you need to build this ramp. That being said, from the information you posted it would appear that you have a fairly small rise and a fairly large radius. From what I can see your only concern is to maintain level along the radial line, I.E. twist. With such a small rise i would think that 3/4" plywood would easily bend to that amount of twist. It would seem that the only thing left is to start construction.