I also run all rope halyards 5/16". I just don't do wire to rope well...You may think of buying a spool of 5/16 Sampson or something like that and learn to do the double braid splice, making your own halyards..big savings.
If there isn't a recent book written about the cost of a free boat there should be. I applaud your intent to revive an old boat like the Helms 25. I have fiddled or fooled with every part of my Helms of course spending more time than necessary. Please let me know if I can help. David #504 Chimera