I think that there are machines (possibly not road graders, though such
function would be useful) that work from either laser referencing or
differential GPS to put the blade under computer control, and ignore the
whole question of where the wheels are at or how long the wheelbase is,
just moving dirt to where it should be. Probably only in the home-build
arena of people that could afford to buy one outright but choose not to.

My impression is that running a typical grader is a very hands-on
operation. What you propose to build is somewhat less so (road drag
might be one name for it - more for road maintenance than original
construction) and there are several very simple variants (which are
limited in what they can do, but also simple to build and use so you
might use them before things get too bad), such as chain link fence or
bedsprings pulled along, or a triangle of 2x lumber with 20d spikes
driven into it (and possibly some weights added, possibly more on one
side than the other - also effects differ depending on orientation of
the triangle to the road as you pull it), or a split log drag (good
stuff can be found in the on-line out-of copyright books/magazines on
that old thing - which is still a good thing for dirt roads. This
wikipedia article has the basic goods on it and links to a patent with
drawings of a more advanced version (more bars and scrapers)

http://en.wikipedia.org/wiki/King_road_drag

The pre-motor graders (ie, horse-drawn ones) would be a good place to
research what worked in the past and start adapting from if you're
looking for more of a grader - though those did have an active operator
(or two) standing on them moving the (control) wheels.

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