Scott,
You say that there is barely enough room to insert the rudder in the casting. I assume that means thay you don't force them in. The bolt that holds them and acts as a pivot should have an elastic stop nut so you don't have to tighten down (and clamp) the rudder. How much force does it take to pivot the rudder? My guess is that while sailing, there will be enough forces tending to open up the castings that you won't have a problem. I have the opposite problem after 20 years of sailing my Hobie 16 and that is that I need shims. I cut them from plastic milk bottles with a hole saw and use what I need. I don't try to overtighten the bolt to squeeze the castings.
Howard