Fireman,
If you have not, you should take this question to the C&C email group, I would think you can find another 25 II owner who can help you out.
I'd expect the third sheave in the base is for the
spinnaker halyard you don't have. You'll
rig that halyard to a single block that has to hang from a crane/extrusion on the mast head that positions the block free to swing and above the forestay. There may be an opening in the mast about 2 feet below the head for the
spinnaker halyard to run internally. The topping lift needs to run to a block about 2/3s up the front of the mast. Again, see if you can get measurements from the C&C board.