My P323 also has a keel-stepped mast. Pearson controlled the vertical deck movement with a tension rod connected from the mast collar all the way down to the mast step in the bilge. I assume its the same in yours.
This means that ideally, you want the turning blocks somehow attached to the collar. Otherwise, they should be deck-mounted as close to the collar as possible.
It should be possible to get (or have fabricated) an organizer plate with a hole in it. Unfortunately, this would require pulling the mast. Perhaps you could make one that is split in two halves, and lift the collar and install it without removing the mast.
Here's an example of one made by Garhauer.