The original specs for a Bristol 35.5 was 5/16 stainless 7x19 wire rigging according to my sources. What you actually have is anyone's guess, but if the sizes don't work very well in standard try remeasuring in metric, someone may have converted over at some point in the boats life. 5/16 would normally be replaced with 8mm btw.
If you don't kow the age of this stuff it may be worth having it dye tested to check for crevice corrosion at the terminals. While it is possible for the wire itself to go bad, nine times out of ten it is the fittings that fail, not the wire itself.
Check out
http://www.energy-tech.com/article.cfm?id=26553 for more information on dye testing. Not that it's full proof, but it should give a very good indication of the reliability of the remaining fittings.
If you do need to replace it, that is a whole different story. Most manufacturers of rigging recommend a 8 year service life untill dye testing needs to be performed every year. Now in practice most people don't follow these recomendations, but it is something to keep in mind.