Cal 33'''' running rig
First, are the existing halyards rope or wire to rope? This is important due to the sheaves (pulleys) in the mast.
3/8" is sufficient size, but unless you are familiar with splicing and whipping, it is probably better to contact Hall or Aramid or a rigger in your area. Check out Samson''s website, New England ropes, Yale cordage, Wichard and Tylaska shackles.
I think that lines should be different colors for easy identification, so I would not buy a 600'' spool. Others would disagree.