:3
General Update
Clutch is (mostly) in, just waiting on allen keys turning up so I can finish tightening all the bolts for definite.
Clutch fork is in. New throwout bearing is hanging out on the gearbox for now, to try and stop me forgetting to put it in place when I refit the gearbox. (Ooh, I'd be so annoyed...)
Gearbox has been drilled and tapped for the last top-cover bolt, where
the casting was broken away. Turns out I didn't quite build it up as
much as I should've, but there's enough there to stop the bolt walking
away. It's also been painted; but will need a little touch-up work
after I've finished wrestling it into place, I'm sure. And then I
stuck a CV boot on the top of it, to act as a gaiter for the gear
lever. 
(That needs hose clamps to hold it in place, but that's no big deal.)
Can't find my gearbox bolts, despite knowing they're in a labelled bag somewhere; so that's the big reason the gearbox isn't in already. New ones are on the way, at which point I'm sure I'll find the originals.
And it turned out that, despite sealing up the rest of the cooling
system, I totally forgot to seal or tighten the top of the thermostat
housing, so when I set up the temporary potentially-total-loss cooling
system (I stuck a meter length of radiator hose on the bottom inlet,
pointed it at the sky, and filled it with water. ) it started to
weep out... and then when I ran the engine, the fan sprays it
everywhere. (So that's why you can see all the spray on the video. No,
it's not fuel! Yes, I did give some serious consideration to what to
do if there was a fire, because there was not exactly a quick egress
path. Yikes! 
I did, since then, seal the thermostat housing better
so that's not leaking any more.
Exhaust bits have turned up, too, so I can make a bit more progress on my headers. That'll be slow going, I'm sure! It'll be good to have the exhaust somewhere behind the bellhousing; that way I can fit the radiator without it being right in the way, then I can run the engine for longer.
And I've spent some time playing about with the ignition timing table in the tune; largely made up from researching the mechanical advance curves listed in the manual for the D200, and a very gentle approximation of the vacuum advance curve from other data online. The table I quickly threw into it for the test-run in the video was very soft, just trying to get it to catch and run enough to provide me something to work with. (By the end of the video, when it was basically up to temp and idling happily, that was at about 10 to 11 degrees.)
Unfortunately, I ended up having to put the 'soft' timing table back in anyway; because the one I made up cherry-picking the data from the manual is too aggressive for this engine. There was no harm done, but there was quite a spectacular fireball.
How much of a difference that should make, I don't know.
What I do know is that the table I made based on the curve data from the manual, and the scant info I had about the vac advance (and I didn't even go as far as the info I was finding had said, because it was quite clearly not the right vac can. There was no way I was going to give it the equivalent of 22 degrees of vac advance, having seen how poorly it behaved previously until I softened up the map)... wouldn't even idle, and liked to fart back through the carb.
It will need a lot more work yet, and I'm sure someone who regularly tunes engines will just have their head in their hands, but it's a starting point!
In addition to that, I cured a mild miswiring fault that caused my ignition/fuel relay setup to not work as expected; and tracked down a backfeed that was causing the engine to continue running when the ignition key is turned off. The alternator was feeding back through the warning light, which was easily cured with a diode.