Greg,
The attached photo is of my 1988 O'Day 322 masthead. My mast says Z-Spars.
The bottom of the pictuer is aft and has the main halyard and the topping lift. The top has two additional sheaves which I believe are for a primary and backup jib halyard. If one has the CDI roller furling, then you don't need any jib halyard as it is self contained in the furler. If you have a traditional roller furler, then one of the two forward sheaves would have the jib halyard.
In my experience, a spin halyard runs on a block attached to a spinnaker bail out in front of the forestay. You see that spinnaker bail at the very top of this photo. If you used one of the jib halyard sheaves for the spin, you would experience chafe on the spin halyard as the spinnaker blows out to the side of the boat. Halyard chafe would cause a disastrous failure at the worst possible time (higher wind). With a block on the spinnaker bail in front of the mast, it pulls freely in any direction from a port reach to a starboard reach. I do not have a spin block rigged.
You could perhaps rig the spin halyard from the head of the spin through the spin block on the bail and then through the jib sheave, and down the inside of the mast, the benefit being the tail of the spin halyard is then inside the mast.
PS - I ordered all new sheaves from US Spar a couple years ago. I may be able to dig out the details from the invoice if you need them.
PS - I replaced that dreadful anchor light, which is one of the reasons the mast was down.
Cheers.
