I have just over 1.25P Dmin to rig the mainsail sheeting.  Below is the planned sheeting arrangement drawn with exaggeration for clarity.  Have I overlooked something?

This post turned out pretty long, skip to the bottom to see the pictures of the new sailplan.  If you're interested in the details of how I came to it, or have time to comment/criticize my design that would be appreciated.

It has been a bit of a journey, but I'm finally ready to start sewing sails for Leeway.  We started refitting her because we thought she'd make a good Junk Rig platform for us, but had some shortcomings.  In the end we completely gutted her, and re-ballasted the boat and moved the fore-mast.  As refits do, things have taken a while but we're finally ready to sew our Junk Sails, which was the idea that started this whole project.  Details of Leeway are on page 1 of the thread, but basically she's a Jay Benford Brigantine (40') that weighs about 28 000lb in cruising trim.  Her original sail area was 940 sqft (87m²).  I originally drew sails close to Arne's, along with working through PJR.  That was early in the refit and what allowed me to get on with moving the fore-mast and then building the rest of the boat interior.

In the end, given Leeways tall masts, that are pretty skinny at the top (4", 102mm), I think I'd prefer shorter/lighter yards.  At bottom below are figures of my preferred current sailplan along with it superimposed with the older 70^o yard sailplan I drew.  I suspect (hope) either would work, but the shorter lower angle yards check more boxes for me.

The forums on the JRA were invaluable, although sometimes dizzying, source of information beyond what PJR provides.  Some of the design features of the sailplan include:

-shorter low-angle yards for less weight aloft (yard goes from 12.5kg down to 8.5kg) inspired by David Tyler's Weaverbird

-like Arne and PJR, all the lower panels are the same to simplify construction

-as per Arne and David Tyler, battens are at 10^o as a replacement for the batten angle calculations for flat panel sails PJR recommends

-Boom has 4% removed as per Arne's instructions to help avoid sheet tangles

-Second batten from the top slightly longer than sail to help avoid sheet tangles

-foremast is raked forward 4^o to help sail behave when running downwind, thanks to recommendation by Annie Hill

-luff of foresail is parallel to the raked mast.  The plan is to use short batten parrels with both ends attached to the luff a' la Paul Fay to control the fore-aft position of the battens.  Given the strong taper of our mast this should help encourage positive batten stagger when reefing.

-mainsail is canted 2.5^o on the mast.  Mostly, this allows a bit better clearance with the fore-sail sheets.  I also like the look given the raked fore-sail.  It also helps the very tight Dmin for sheeting the main.  This probably also gives a bit of anti-twist to the main (more balance higher up), which is good given the steep sheeting angles make it harder to control the top of the sail.

-Both sails have split lower and upper sheeting.  A single sheet system can make it hard to control twist in the top of a high aspect ratio sail (David Tyler) and Arne has used split upper and lower sheets on the schooner Samson successfully.  I'd prefer a single sheet per sail, but we'll start with upper and lower.

-Comparing the 45^o yard to the 70^o yard, you end up with a bit more area in your "storm canvas" as the area of the top two panels increases from 11m² with the 70^o yard to 13m² with the 40^o yard.

The finished sailplan is 97.8m² (1050ft²), compared to 99m² (1062ft²) of the 70^o yard or the 87m² (940ft²) she started with as a staysail schooner.

Thanks to everyone, including those I've forgotten to mention whose contributed to improving my understanding.  I think I'm now at the point where I have begun to know what I don't know :-)

Graham, I think we have a fair bit in common, both in design philosophy and in the benefits/drawbacks of metal boat ownership.

There are times when plug welds are the only solution I've been able to come up with.  As long as the slots are wide enough, the welding isn't difficult with Mig.  The plug welds in the mast step were trickier as the thicker plate means you have to pay a lot more attention to make sure the first pass is fusing both the the lower and upper plates.

You are right about rebuilding the keel being a nightmare (well maybe more like a very uncomfortable dream).  We had steel punchings/concrete for ballast in an aluminum boat with only an soft epoxy coating on top.  In the end I found circular holes being galvanically drilled in the aluminum by the steel punchings.  After removing 10' of keel, removing 5000lbs of concrete/steel, welding new keel plating and stacking/pouring 8000lbs of lead, it's not I job I would care to undertake again.   In the end we now have better ballast that is down lower.  Because the lead is more dense, we used a double wall and an airspace to separate the ballast and we now also have integral water tanks above the ballast in the keel.  I also have an oil reservoir a bit above waterline that feeds into the ballasted section of the keel.  Any further failures will result in a slow leak of oil rather than corrosion.  I just have to decide on an appropriate biodegradeable or inert oil.  Silicone oil seems like a good choice that would allow further welding, but is expensive.  Biodegradeable hydraulic oil is also an obvious choice, but I worry what it would be like to deal with if I ever need to weld on the keel again.

I've finally completed Leeway's mast partners.  We are refitting the entire boat and sometimes starting one thing leads to another and so the mast partners took longer then expected.  Access and the most logical workflow had me complete our chain locker, rode locker, windlass mount, anchor rollers, saltwater washdown outlet and some deck cleats as part of building the mast partners and mast step.  I've added some photos to my album for those that are interested and will describe what we did below.  Before that, thanks to all who contributed to the thread and helped guide the process.  

Our boat came with freestanding 10" diameter aluminum masts.  The mainmast was in the right position so it was left in place, although the mast step had to be rebuilt as part of reballasting the boat.

Our foremast from the previous staysail schooner rig, was not in the right location and we moved it as far forward as possible while maintaining adequate bury below decks and keeping the mast step within the boats waterline fore-aft.  The new mast has 4 degrees of forward rake.

To reinforce the deck I added a 1/4" (6mm) double plate.  It has eight slots cut in it so that it can be plug-welded to the deck, as well as welded around its periphery.

The doubler was 1/4" because that was as thick as we could reasonably get to bend to conform to the camber of the deck.  Even then, temporary chocks and wedges are needed to coax it into place.

I was worried about warping the deck, so I added some temporary reinforcing below deck and carefully skip-welded the entire doubler in place.  Only after the doubler was fully welded did I cut the hole for the tube that would form the mast partners.  The cutout is slightly inside the weld that joins the double plate to the deck.

I had the mast step temporarily in place and used a laser level and a Tilt Box II to precisely align the mast with 4 degrees of forward rake.  The tilt box is really useful when you want to align something that is not going to be level.  After the tube for the partners was welded from above I went below to make the lateral webs to support the mast.  A bit of careful template works yields pieces that fit precisely which makes the subsequent welding much easier.

Finally the mast partners are complete.  All the webs are 10" high against the mast tube.  The lateral webs extend 24" to the hull sides, while the fore-aft triangular webs are 20" long fore and aft the mast.  All are made from and capped 1/4" plate.  The white tube to the left is the chain locker that was installed at the same time.

Having the webs to support the tube-of-the-mast-partners below decks leaves the area above decks relatively free of toe stubbing obstructions

The mast step is made primarily from 5/8" plate.  I drilled and tapped holes before putting in place as I was growing tired of working in the 2 square feet of cramped space available to me in the bow!  The keelson was extended to support the bottom of the plate.

Before the top plate was welded in place, I welded the vertical end-plate inside and out to the keelson as well as the 3/8" doublers on the hull plate.

After the top plate was welded to the doublers on the hull, the end plate and plug-welded to the keelson, then the aluminum extrusion was bolted to the plate and the mast step was finally done.  It was a lot of welding in a very small space.  I managed to set myself on fire twice and my knees are still recovering despite wearing knee pads.  However, I'm pleased with the final result.

Thanks David and Graham, that helps.  Graham, I think I should be able to implement webs of the scale you have, but put them below decks, so it is nice to know what you have is working.

David, I like your implementation of the arch that extends all the way to the topside stringer and I will extend the triangular webs for Leeway such that they continue to the topside stringer.

David it looks like your reinforcing arch is aft of the mast partners.  This makes me wonder if I might simplify things for myself by attaching my triangular transverse webs to the aft 1/4 of the tube that forms the partners.  If you look at the location where the tube goes through the deck, the existing 2" angle deck-stiffener passes slightly ahead of the mid-point of the tube.

Thus, I either have to remove this stiffener to make room to weld the triangular webs, or I could attach the transverse webs slightly further aft on the mast-partners-tube and leave the existing deck stiffener in place and welded to the side of the tube.  My first instinct was that the transverse webs should be located on the midpoint of the sides of the tube, but if they could be moved further aft that would simplify the installation and allow the additional stiffening of the existing framing to be left in place as well as adding the triangular transverse frames (see Option B below).

Any thoughts appreciated.