If you go for an all-line halyard, the new materials are strong enough to end up being thinner than the wire was, just as strong if not stronger, and with less stretch (which is why the wire was originally used). Don't get line that thin: it will hurt your hands pulling it up. Larger diameter line (spectra, dyneema
, etc.) will be stronger than necessary, but more comfortable on your hands. If you get the same diameter as the line on your old halyard was, it should fit the masthead sheaves with no problem. Spinning the sheaves with some sandpaper in the groove should take care of any burrs.