With the shrouds aft like that you needn't worry about the backstays. Slack them completely, your shrouds and forestay will support the mast. Use the shrouds to center the mast and then adjust the backstay legs. One problem you may encounter with wire that big is that the tension required to keep it from being downright sloppy will be considerably higher than the original tiny wires and you may banana the hull. This will be especially true of your backstays which are already going to be only lightly loaded (relatively) which is compounded by them being doubled.
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