I decided that the spinnaker halyard would be better mounted higher up the mast. It was attached to the stock forestay mount which has three attachment points built in. I don't know what they were designed for.
This photo shows the forestay attachment to the fitting. Also shows the upper swivel for the roller furling.
My installation of the tang for the spinnaker halyard is probably overkill. Those are #10 socket head cap screws not pop rivets. They were left over from installing the window on the Balboa.
The installation was a complete success! Jibing is easier with the extra space in front of the forestay and there is more adjustment available for the front edge of the spinnaker.