This is where most ramps stop - The angle. This is created for you, the one time ramp builder so you can mold the angle 6 inches at a time. In my early days of building ramps, this is exactly what I used to create my Rev.1, Rev.2 or Rev.3 angles.
I came up with this so you don't have to find a shop that can bend the metal to my exact angle. Having metal bent for 2 side rails would easily add another 200 bucks to your final ramp costs.
NOTE: This is not on a constant radius, it is an Elliptical Radius. Yes, it takes longer but it is worth it. Be sure to match this up to your cardboard drawing. The goal is to have the final angle within 1" of the specs from top to bottom.
I advise you to use .120 wall tubing. [1/8"] If you are using a stick welder, you MUST use .120 wall square tubing or you will probably have holes burned through at every weld. If using a Mig welder with gas you could use .065 wall square tubing. If you don't know the difference between the the welders I just mentioned, don't go any further. You are already over your head.