I did something similar with the topping lift on my Nonsuch 26. The padeye at the end of the wishbone (boom) is on the outside of the end casting, so that the shackle was levered sideways against the casting. To get a better lead, I used a Dyneema link.
I made a loop of 3mm (1/8") Dyneema, about 36'' in circumference, joining the ends with an end-to-end splice. This was doubled and put through the padeye, then the end loops were brought together and attached to the shackle. This gave 8 parallel turns of cord with a breaking strength of 2200 lb/turn. I wrapped the whole bundle in rigging tape to reduce chafe.
This method means you would have to use shackles to attach the lines to the Dyneema link, but you have the shackles already, and it's neater than knots, with no chance of working loose.
The setup is in place for about 5 months of the year, then disassembled and inspected before being put away for the winter. The cord is going into its fourth season with minimal chafe, but I don't know if a lifespan of a decade is realistic. I certainly recommend regular inspections, although that's harder in your case than mine. At any event, the cost and effort of making a replacement are minor.
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