So that this doesn’t drag on for too long, I picked up a 5-speed propshaft from my local supplier (capri_bits2 on eBay) for a very modest sum. It’s not had the fine misting of oil over it for all its working life, like the old one, so it’s a bit grottier on the surface, but all the joints seem fine and have no play (that I can tell). 
The propshaft for the old Type-H box is on top and the one for the Type-9 is below it. The older one is about 2cm longer…
These are the ends of each prop that would mate with the tailshaft of the gearbox – Type-9 on the left and Type-H on the right. Not a cat in hell’s chance of mixing them up then. 
After ridiculous amounts of blood, sweat and tears whilst wriggling around underneath the car, I finally managed to get the new box on for a trial fit. 
I really need to be able to get the car a couple of inches higher to do this sort of thing – the gearbox won’t fit onto the trolley jack and slide under the car, so you have to drag it along the floor then manhandle it onto the jack … in less than a foot of space underneath the car. 
I might put a wanted ad up in the Post Office to see if any of the local farmers can do me a couple of railway sleepers cut in half lengthways. 
It’s not all bad though – the gear stick lines up pretty much bang on in the hole, just like you said it would Gustaf 
As you can see from the earlier shot and this next one, though, it’s not the “right” gearbox crossmember that determines if it fits … I think the strengthening piece in the tunnel (that the crossmember bolts onto) is simply set further back on cars with the later gearboxes.
As suspected, I need to extend the crossmember forwards to meet up with the existing mounting points, as I’m not going to just drill new holes in the tunnel and hope it doesn’t fall out later. 
Today’s question, then, is how do I know what height is right for the back of the gearbox to sit at, so I know how deep to make the spacer? 
dangerousdave: Well done stu 
an easy way to move engines and boxes about is on an inspection trolley thing which you lie on and roll under the car too late now though
As for angle, set it so the engine is as flat as possible with a slight tilt backwards, or put it where it was before as you will have pics of your engine bay, just get the cam cover the same distance from the heater bubble
me: Not even enough room for that … been there and got the t-shirt The bellhousing will just fit under either the front crossmember or the nearside sill, but only if you drag it along the ground. 
Thus the idea of the railway sleepers to create a kind of mini pit.
olly22n: Beware, i had no room, so jacked the car up higher with the aid of blocks of wood, but then found my jacks weren’t high enough to lift the damn box up
therealpinto: Ideally the angle of the engine/gearbox should be the same as the angle of the rear axle, meaning the prop flange. This isn’t always easy to get spot on (you may need to space the center bearing down too) but try to get it as close as possible. Otherwise you may get some vibrations.
Also check for clearance in the tunnel, you may need to lower the gearbox a tad.
I’m told that a MkIII-V Cortina gearbox crossmember will “fit” but that’s second hand info.