Here is yet another way to build guage wheels which is what I just finished on my 72" KK landscape rake. I purchased 2-10" pneumatic swivel caster wheels from Harbor Freight ($9.99 each on sale) and bolted it to a 3/8" peice of steel. Then, I welded a 1" nut with 1" All-Thread to the top side of the plate. The top is held in place by more nuts, washers and lock washers.
The part connects to the rake is based off of other posts I've seen here and uses a short peice of receiver 'scope' tube.
The main advantage to this approach is that each side is infinitely adjustable without taking it apart. The downside, though, is that I need a big wrench around to do that, and I need to keep the threads lubed up to keep from rusting. As other's have mentioned, this design can be used on box bladed and other implements where you want to maintain a specific depth.