Finally got round to sorting the shocks.
I was going to shorten and re-thread the rods but after cutting 3/4" off the springs, the rods are still (just) holding the springs in tension on the softest strut setting, so no need to shorten.
The cut-off ends of the springs go under the top shroud of the shock, but that area needed further modification to make sure that the springs seated centrally, so I had to make a 'top hat' spring seats out of brass and a steel seating washer.
I'll find out where the ride height is in another week or two, but from a little trigonometry I've done with the rear end that shouldn't have lowered it too much, and I do have the option to wind up the preload (max preload now will be about the same as minimum was before). I'm expecting the back of the bike to be lower by about 1.5".
If it’s too soft and bottoms out, I have Plan B – found a supplier of 255mm shocks. At about $160 a pair they’re cheap shocks, but still £80 I’d rather not have to spend.
Bottom photos show the assembly without the top chrome shroud in place to to show how it fits together.