[TomP]

Yes, you can replace rod with wire. But it is not really that easy. There are many things to consider and they add up quickly, both in cost and wieght.
Here is a partial example of a re-rig on a 1981 C&C 36 IOR that we recently completed.
Backstay. lenght 51 feet.
Nitronic 50 -12 7mm dia, 12,500 break, .21lbs/ft, $7.00/ft
1x19 Wire 3/8 10mm dia, 16,000 break, .30lbs/ft, $3.50/ft
Dyform 1x19 8mm dia, 13,500 break, .23lbs/ft, $4.90/ft

This is just the wire, you must consider the attachment points (terminals, turnbuckles, etc). Typically the termination pins will not match, for 3/8 wire you could use a 5/8 or 3/4 stud, the corresponding turnbuckle would have the same pin (5/8 or 3/4). Dyform gets you close using 8mm you can use a 5/16 terminal which is available in 1/2 or 5/8. IF there is sufficient meat on the chainplate, it MAY be possible to drill the hole to 5/8.
You must consdier the weight when doing something like this. You must examine the affect of the weight aloft will have on the righting moment of th yacht. In the example the rod weighs about 11 pounds, the wire is 15 pounds, and the Dyform is about 12 pounds. The four pound difference between rod and 1x19 may not seem significant, but add the associated terminals and the rest of the rig you could be swinging an extra 60 or more pounds aloft. It could make an otherwise stable boat into a rocking chair, not to mention the safety factors. Get a copy of "The Apprentice", "Skene's Elements", and if you really want to hurt your head, Marchaj "Aero-Hydrodynamics of Sailing" and study righting moments and rig design, find a knowledgeable rigger who has done similar work.

Strength is always more important.

Hope this helps in some foundation.