Are you replacing your sheaves though? I was under the impression that was a must as 30 years of wire wear is not going to leave the existing sheaves in "rope friendly" condition. I am also under the impression that the direct replacement sheaves are suitable for rope and wire.
I would be tempted to go with a higher grade line (VPC, T-900) and go down to a 5/16" diameter.
I read a wise quote once: "We'll spend thousands on sails, but are willing cheap out on the lines that their shape depends on." Food for thought.
