You might find some guidance on this thread --  www.catalina36.org/forum/sailing-discussion/masthead-rig-spinnaker -- which guided my installation 7 years ago (and why I appreciate this forum, too).   As in that thread, on our Mk I, the assymetrical spinnaker halyard exits the mast a couple of feet below the masthead (the exit plate on our boat is on the starboard side; in RJWilson's photo in the post below this one, its on the port side) and runs from a cheek block to a block hanging from the the bail at the masthead.    On our boat, the bail was already there (I believe this factory standard, from the Catalina drawings) but I added the 2 blocks, and a halyard clutch a couple of feet up from the base of the mast.   I dropped a messenger line from the exit near the top of the mast, with maybe a foot of sash window chain for weight at the end, and had an assistant fish it out at the exit near the base of the mast.  

I've got some pictures below.  As you can see, I also added a winch near the base of the mast; I don't need it to haul up the asym spin, but that halyard is very useful for hauling up other things, such as loading the the dingy on and off the foredeck, so its handy for that.  I've also added a couple of small cleats near the base of the mast to secure the control lines for the spinnacker sock.

For the tack line, I added a small block shackled onto the anchor roller assembly, and we use the ATM "Tacker", which is a plastic taco-shell-like gizmo that fits around the furled job.  The tack of the asym spin shackes to it, and then the tack-line as well.   The tack line runs through fairleads back to the cockpit so that we can adjust as conditions require.   The halyard does not, as raising and dousing the sail requires someone to be on the foredeck anyway (all of our other control lines run to the cockpit - see www.catalina36.org/comment/55992#comment-55992).

Note that in RJWilson's photo (in the post below this one) the setup uses a fairlead above the exit plate (we installed a cheek block, which might cause slightly less chafe).  In either event, it is ESSENTIAL to have something installed in that location to keep the halyard pulled somewhat aft, so that it can't get caught up in the top of the jib furler and cause the dreaded halyard wrap.   See www.catalina36.org/comment/41960#comment-41960 for what a halyard wrap looks like!
     In addition, we've adopted as standard operating procedure to make sure that whenever the spin halyard is not being used, the shackled end (i.e., the end that would attach to the head of the spin) is lead around to the aft side of the spreader and secured there, and tensioned, to make sure that it cannot get caught up in the jib furler.   I learned this the hard way a few years back, when after a nice day-long sail I went to furl the jib prepratory to entering a harbor and discovered a halyard wrap from the assym halyard.  Had to climb the mast in a bucking sea with a madly flogging genoa (as we had to luff up into the wind) in order to undue the wrap.   Never again!

The spin halyard is the one that exits the mast a couple of feet from the top. My lines are all ran back to the cockpit.

Shouldn't the spinnaker fly outside the forestay?  The factory position for the spinnaker halyard is a block attached to a bail that is screwed into the masthead fitting. This puts the spinnaker halyard above and outside the forestay.  The bail is just held on by 4 screws which go into the masthead fitting.  It doesn't seem like a too difficult of a retrofit.  Putting the halyard below the masthead would make gybing the spinnaker easy as long and the headsail was furled prior to setting the kite.  The headsail sheets would need to be kept out of the way too.  It is a nice cruising setup, but without a headsail, you will slow way down while setting the kite.