I helped my brother build one about 15 years ago from various spare parts. He had a Ford 1700 (~25 pto hp), and it was all it could do to run it. But, we didn't do much in the way of gearing experimentation. As I recall, it was a simple design with a 3 pt hitch connected to a piece of 8 or 10 inch channel oriented perpendicular to the line of tractor travel. Power was transferred by shafts from the PTO through a 1:1 90 degree gear box to one end of the tiller. The tiller was maybe 4 or 5 ft wide, so it was offset to one side to allow a slight overlap on the previous pass without the tires traveling over the previous pass. We just used standard garden tiller tines bolted to steel plates that were welded to a shaft that was the full width of the tiller. A chain was used to transfer power from the upper shaft to the lower. The tines worked fine, but wore at a rather rapid rate. Unfortunately, I don't have any photos. I could probably sketch something out if all this description doesn't make sense.
