Just this past weekend, I helped a friend of mine replace the rudder on his Catalina 30 in Noank, CT. I know that this project took a lot of time that I was not around to learn from, but my help was needed when it came time to install the new rudder. Hopefully my account of this project will shed some light on the task at hand. A couple of notes here were that my friend had asked the marina to leave his boat in the sling of the travel lift at a height that made it possible to lower the rudder and its attached post enough to clear the ground. For his Catalina, we needed about 3 1/2 ft. in order to lower the rudder down enough to clear the post. Another note was that he made a blocking to hold the rudder in place, using the blocks around the shipyard to meet the height of the rudder once it was in place. Special care was taken to ensure that the blocking was secure and would NOT fail as the boat moved in the sling when getting and off. This was accomplished by nailing the blocks together with a couple of 1x3s, and the rudder was shimmed up snug for proper fit.
With regards to the replacement rudder you purchased on EBay, I am assuming that it has holes drilled into it already. It may behove you to see if these holes align with your quadrant and cap early on. We had to dry fit, mark for the quadrant bolts, remove, drill the holes, and then reinstall in order to mark for the cap holes.... Just remembered that we also had to dry fit for the proper measurement for the length of the rudder post. He also had a lot of grinding to do in order to get the rudder to turn fully in both directions.
Please do send pictures so that we know that our suggestions apply......
Good Luck to You