Sounds like the line was doubled up and then "served" and perhaps some decorative Turk's Heads laid on. One might consider carefully disassembling and lengthening those lines and then re-serving them.
I assume that you will be adding some other type of stanchion or such as to fairlead the lifelines away from your winch and that the end result will not be loose lifelines?
I would be probably asking the question of how the boat was sailed in it's present condition. Was there always a clearance problem? There are details here I am not getting or understanding.
Re-reading your OP I see you wish to lengthen the lines. I would utilize three strand dacron, maybe nylon, as you can splice it easily and utilize the stretch factor for tension. I would, upon tensioning and securing the end, whip the parts together with sail twine. I would then serve it with strips of canvas, which will protect the line from UV and other damage. I'd then apply a few turk's heads to secure the canvas and provide a neat appearance. I would recommend a hard laid light sized manila for the turk's heads. It will not deteriorate under UV light as quickly as a synthetic will. An application or two of varnish will serve to protect and firm up the turk's heads. The exposed canvas can be whitewashed for a quite yachty appearance.
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