"Below the turnbuckle" is throwing me. If it's the main halyard, I'd drop the mainsail, cut the shackle, sew a new halyard line to it and draw it back over the mast head. I would assume that the new halyard had an eye splice in it ready to take the shackle. If not, my favorite knot for this application is an anchor bend. Shop around and you can get Vectran for less than $1/foot.