Line notes
Lots of us have made this change over the last decade. Determining factor seems to be the sheave size at the top of the mast. Some older boats will have a narrow one for wire, some have a double-groove one for rope and wire, and some will have a wide enough one for reasonable rope diameter.
This is a situation where "size matters"...

Almost any good quality low stretch line will be strong enough, but (!)... it has to be large enough to handle without cutting into the hands of the crew.
Also, it needs to work/grip in your clutch stoppers (if you have 'em) as well as the existing rope tails on your wire-to-rope halyards.
In real numbers, a 3/16 or quarter inch line would have more than enough strength, but you would probably much rather handle a 5/16 or 3/8 line, for the sake of comfort to your hands.
Sta-Set X would be a good basic line choice, or move up to T-900.
Note that, with experience and study, you can build a "tappered" halyard with only the part that goes thru your housetop stoppers and winches being fat. I have not done this, but riggers turn these out all the time.
In effect, you can create the effect of the sizing of the old wire-to-rope combo, but without the meathooks and with a lot less weight aloft.

Sometimes, technology can be A Good Thing...
Apropo of nothing in particular, we went with 5/16 T-900 on our 34 footer. I tried some 3/8, but it is really a bit too snug going over the sheave at the top of the spar. As to strength, the 5/16 is rated at 7000#...

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Good luck on your project.
Keep us posted,
Loren in Portland, OR