There are a couple of ways to take care of this problem. 5" isn't that bad. First, the motion of the pylons in the castings. You will hear people tell you to epoxy them into the castings. This will give you a great custom fit between the casting and pylon. I don't like this idea as it makes removal more difficult. I like making shims from old soda cans to insert into the gaps. This allows easy removal and adjustment.

Second, tramp tension. This will help tighten up your entire boat. The tramp adds rigidity to the frame like a gusset in a 90 deg joint. There are posts on this site and the hobie site abotu using your main blocks to get some prebend in the frame.

I think you will notice an improvement with just tightening the tramp. Doing the shim/epoxy to the pylon casting joint will help reduce wear caused by the parts moveing against each other.