Robert,
You answer is good, as always
You are right on the specs too (at least that's what I read).
However, Defender catalog lists 1/4" 316 wire to be rated 6900lbs. Still enough but does not leave a huge safety margin. 9/32" is rated 8700lbs - that should be plenty. If what I have currently is indeed 5/16" - thats an overkill (and also a lot more expensive).
There is additional issue with chainplates - the holes on chainplates are much larger than current pins and holes on turnbuckle jaws. From inspection it appears that these holes are original (or if not - drilled very professionally, but for what reason I do not know). Interestingly, the holes on chainplates (and also on the eye fittings on the wires) are about 1/2" (and pins are only 1/4" or some 3/8" or some such on both sides). Holes on headstay fitting and backstay fitting are smaller (3/8"?). So I've got a mix of things going on there.
So, part of this excercise of replacing
rigging is to get properly sized fittings (may be not
turnbuckles themselves, but certainly eyes and pins that would match the chainplate hole size).