Drilled a hole in the gearbox, today, for a reverse-light switch.
Then I drilled another one, in the right place. 
Measure three times, still cock it up, and all that.
Anyway, now I just have to make up a little retainer plate to hold the switch in, and cover the hole I drilled in the wrong place. I might even tap some threads into that hole, just so it looks like it's actually for something, in case anyone ever looks in the future.
glenanderson
Tap a thread in it, then wind a suitable grub screw in with a bit of loctite. No one will ever know. It's not like it's on the internet or anything.
Frankenhealey
Don't listen to that glenanderson with his techno-solutions. You need a REME quick fix, whittle a stick to nearly the right size and wallop it in with a lump hammer. Proper job!
A Few Days Later...
Picked up a couple of my BSP taps from work, so I could tap that additional hole and put a fitting in to make it a breather. Completely forgot to actually do that, though.
In the meantime:
The technical details on the crank/cam trigger sensors I'm using:
It's a Ford 6C11-124073-AA cam sensor, as used in Transits, Peugeots, and whatnot. Figured I'd be best off using an easily available, affordable, designed-for-the-job sensor. We'll see whether or not it's actually stable enough to handle reading a 36-1 trigger wheel at the sorts of engine speeds I'm expecting. (3400RPM absolute max "Captain! She cannae tak it!" levels)
(To be honest, the only reason I'm using this sensor is because I had a used one from work; pulled it out of an engine that had a loss of cam phase signal, replaced it with a new one, and then discovered the actual reason that there was no cam phase signal is because the cam wasn't spinning. Then I bought two new ones once I'd tested that the concept was sound. They're in the region of £20 apiece.)
And I'm using the 1W7T-140464-MA connector, except it's from Simtek where it's a CON-366C.
From the back, pin 1 on the left, this looks like:
- Pin 1: +12V
- Pin 2: GND
- Pin 3: Signal. GROUNDED WHEN ACTIVE, floating otherwise. (I put a 1kOhm pull-up resistor to +12V on this line.)
I write this all down, because it took me an hour of fiddling about
testing every combination of wires because apparently I never wrote it
down anywhere useful, the last time I did all this trial and error for
my shoddy test-setup months back. 
Maybe this is useful info for other people who need a cheap, plentiful
option for an automotive hall-effect sensor; might as well take
advantage of all the money someone like Ford spent on R&D and
mass-production, right? 
Also, yes, the engine turns over that slow. Part of it is that it's still very, very tight; with fresh rings, fresh rope seal, etc. Part of it is that the starter's rather tired. I am definitely thinking a gear-reduction replacement, if/when I can ever get the money for it. Could have the original starter rebuilt, buuuuuut I could also save that money and put it towards one that should turn the engine over faster without working as hard.
Also also, since that video was filmed, I have discovered how to rescale the tachometer on the tunerstudio display, so that it's got an appropriate range.