You are a smart man Lance!
nice application! there are commercial things like this, but making it with a arduino is a lot smoother
and a lot smarter and I would reckon a bloody lot cheaper
John Deere sells a system that can do this, using DGPS and a portable base station, but the setup is around $40,000.
I wonder how much in dollars you have spent and what sort of hours input? How many hours would it take now to put one together? Could we get any pictures of the install?
So there goes the lunchbox heh!