Hi 73bonny, The outer spline (clutch hub) you see is the spider. If back side of spider spline end is worn, it indeed will move back & make it even with the small hub.
Indeed the washer normally only presses on the spine tip. Remember that spline is the the inside of the spider. When spider is forced back with the washer & nut torque it puts pressure on small hub which presses it tight onto taper.
Whatever you decide is what you'll do, that's fine by me.
On a side note the 3 row chain is much more sensitive to chain line than the earlier 2 row. Factory recommends setting chain line + or - .005". Shims are available in .010" 70-8038 & .030" 71-2660 thickness. They go between the 71-2663 spacer (that's against bearing) & the sprocket.
It advisable to use least shims as possible, meaning don't stack three .010" to make .030". Use a .030" instead. These are hardened, not simply a thin washer. Can be a little hard to come by. Many sellers don't stock them.
Lapping main shaft taper was needed, but will move basket inwards. Wear in thrust washer for basket or flange of small hub will move basket inwards. Always measure basket position with clutch plates installed & springs tightened. This holds baskets to the rear (or the right side of machine if you will). The basket normally has some runout on the sprocket teeth. So measure in several places & average measurement. Since basket moves to rear as thrust washer wears, if you have to choose between outwards or inwards to edge of spec, I go outwards as wear will move it in wards.
If you don't actually ride bike much not so important to do all this correctly. However if you actually want to ride the bike & have things last longs as possible, all this stuff matters. Some just ride to the coffee house once a week. Maybe put 150 miles a year on bike at most. It depends on what you do with bike.
Ducati is not a simple bike. However many times easier to work on successfully in my opinion. Triumphs have many idiosyncrasies that will bite you in the rear if you put many miles on bike & don't do things right. Huge problem with Triumphs is 90% of the repro parts are not to spec, even though supposedly made to original blue prints. Not like the old days when you went to dealer & got the new genuine parts. Bolted them in & you're good to go. Not like that at all now.
Going electronic rectifier/regulator is a good plan. What ignition are you using? Trispark has some compatibility issues with some regulators. Look over Trispark web page under troubleshooting. How do you diode is bad, did you test it with ohm meter or the like? Should you need stator, 3 phase high output is recommended. That needs specific 3 phase regulator. Stuart recommends getting 3 phase regulator at this time in case you should upgrade to 3 phase stator at later date. 3 phase allows low speed city use & the use of modern bright headlamp without discharging battery
. Low voltage at lower rpm like in city has been a long term issue for Triumph until they came out with the 3 phase stator.