It really depends on how much you want to spend. The ideal would honestly be to replace your mast head crane with one that has more sheaves to install a halyard, possibly with a spinnaker crane. Run the new halyard internally, and drill a new exit plate in the mast. For the tack, add a retrofit spinnaker pole (I like the one from Selden).
This will also be the most expensive by far.
Instead of the new crane you can likely install a spinnaker bail (see Masthead Spinnaker Bails
) it is still much prefered to run the halyard internally, but if you can't, these will act as a location to add a block.
Either way a spinnaker should always be flown above the forestay, and outside of it. Otherwise you will have major chaffing issues as the halyard tail is forced outside and around the stay attachment points. A-sails in particular need clear runs, because they are pulling to the side, not just forward. If the sail can't fly free of the stay it will defiantly rub there, and you will spend a lot of time either replacing the halyard or the head patch of the spinnaker.