You will probably have to go up there and look. Masthead sheaves on boats that old may be set up for combination wire halyards with
rope tails. They would have a rounded or flattened groove in the sheave for the
rope, and then a narrower "gutter" in the bottom or middle of the groove for the wire. If that is the case, they'll probably
work using an all-fiber
halyard. If the grooves in the sheaves are straight-sided, or "V"-shaped, they're for all-wire. Running fiber in them will tend to make the fiber jam in the "V" and wear out the line. Depending upon the diameter of the line the sheave may still
work, and the wear may be acceptable. In other words, it might take three weeks to wear through the line, or have it jam halfway up, or it might last ten or so years with no problems. New synthetic lines are so strong that the diameter to use is more dependent upon how it feels in your hand than what the load is. You want to be able to hold and pull the line without it cutting into your hands, so 1/2" is a likely size. Getting new sheaves and bearings up there would probably be worth climbing the mast to do it if the existing sheaves don't have a rounded or flattened groove