Hi there,
You were good right up until the assumption that the (running) backstays would take something like 25% of the load. They don't. They just keep the mast from popping forward when sailing with the mizzen stays'l (you do have one of those, right?), and possibly when hitting a stiff chop. The weather shrouds will handle all of the mizzen loads, and almost all of the stays'l loads, so we're talking a tiny fraction of the total load here. Put up some 3/16" 12-strand Spectra. Hang a Colligo purchase system at each lower end (
http://colligomarine.com/products/co...rchase-systems). Belay to a PXR cleat. All you need.
Oh, and 1.5 to 2 isn't much of a safety factor...
Fair Leads.
Brion Toss