Roger -
Starting with a clean sheet of paper .......
Seldén probably makes the best tang system in the industry. I have an older design SeldTang system on my boat .... Ty37 Bob Perry design. In most of the rigging for this boat I keep back calculating a Safety factor approaching 4 (probably because the rig size calculated is just under 3 and he used the 'next larger' available wire, etc.) It is apparent to me that Perry (and probably most others, including yourself) determine rig strength by pulling a theoretical force parallel to the horizontal plane applied to the mast head and calculate the moment arm about the 'full load' metacenter with the boat held at the ~45 degree angle. I did actual heeling/force testing (not to 45 degrees over) .
I have the 'old' style of SeldTangs and never doubt them as most boats that have them are multi-circumnavigators, so that should give you some confidence in sailing in the high latitudes. Although these are usually quite expensive, it might be a good solution .... buy & install, including the 'lowers' connections as I deem this to be equally important strength-wise.
We've discussed in your ‘chainplate thread’ the need to minimize all stress risers, bends, etc. and make all the critical parts 'straight' or inline tension, without flexure ... all to lessen fatigue endurance failure. We also discussed the importance of surface finish, etc. to lessen fatigue propagation.
Starting with a blank sheet of paper plus your mast .... below is my interpretation of the ‘SeldTangs’ for local manufacture whose only 'tricks' would be the special eccentric spacers that are milled from bar or rodstock ... whose interfaces at ~10 deg from the axis of the bore. These spacers will eliminate virtually ALL indeterminate flexure in the tangs proper. The bolt length increase to accommodate the fixture probably would need an 'auxiliary tang' to help support vs. flexure in the bolt .... my sketch is bent but I dont deem this to be critical in this particular member as it only there to as a 'helper' to reduce the bolt stub-end from flexing as a 'cantilever' .... the helper tang removes the (weak) 'cantilever' action so the tang put you into supported beam formulas instead of the 'inherently weak' cantilever requiring 4X the strength of the bolt in flexure - changes the weak cantilever to an end supported beam (mechanical/stress advantage = 2+). Stub ends and cantilevers are too unpredictable and vulnerable for my 'conservative' design mind. Ill leave it to you to calculate the lateral flex and shear in the bolt stub (using standard beam flex calcs.) at full rig wire load (including FS) to see if indeed you actually need this aux. tang. Seldén in their ‘simple’ tang systems usually has this 'aux' tang to support the stub end flex. .... I alway did this when engineering gantries and overhead cranes, etc. (about 35 years ago - groan).
The 'spacers' would be milled from rod stock. Simple operation to mill @ 10 degrees (the probable mast intercept angle). Thickness of the spacers would be critical for clearance to mast, clearance to bolt head, and clearance for rigging eye in between the two tang plates. A bolt loaded at 10° off, isnt going to put that much of an axial thrust load against the bolthead ... but you can do the trig. to verify as Wire load X Sin10 = 17.4% of wire load. For the tangs proper, just for 'spits and giggles' Id increase the FS by ~17% because of the ~10° eccentric bore holes - just a SWAG.
Forgot/edited - You'd also need to consider to apply a small ferrule over the thread contact area of the bolting on the thread end of the SHOULDER bolt - just in case the torque/strain relieves the pressure on the friction joint.
Summary of my 'cobbled' design:
•No flex in tang plates – reduction/elimination of stress risers
•Needs milling of the 'spacer faces', spacial ‘stack up’ of combined length of spacers is important
•Torque to 80% creates a 'friction connection' against the compression ferrule
•Compression ferrule creates large cross section to prevent ‘mast bore compression failure@ projected load bearing surface’
•Aux. Tang is ‘helper’ to lessen flex in bolt stub ends – can be bent.
• As in all fatigue service applications, surface finish will be important - mirror polish.