Your looking real good with your project. They make different thickness of clamps that attach to the cylinder rod to limit the travel. Basically 1/2 round pieces of aluminum with a flat spring to hold them together. These would be inexpensive, adjustable and available at most Farm 7 Home stores. Down travel can be limited with a chain or rod between the cylinder yoke and the bar, also adjustable.
90% of the mowers use a rod with an eye in one end and threaded on the other to hold the bar vertical. Threaded end goes into a hole on the bar with a nut with a rod about 4" long welded to it. Can turn loose that way.