The rear, however, is a different matter. Even with GAZ rear coilovers wound down as far as sensible, mine sat far too high.
First 'solution', quick and dirty : move the top mounts to above the shell.
From this :

To this :

Using these bits :

Here's what it looks like from inside :

Moving the top mount like this drops the car about 13mm; later I put some spacers under the mount, increasing the drop to 20mm.
Still not enough.
A proper roll cage was on the agenda, so I took the opportunity to perform a little surgery on the shell :


This raised the top mounting points 60mm above their standard position. Suspension travel was now limited by the wheel touching the arch, by the rear spring perches touching, and by the ARB drop link hitting the underside of the car.
Careful work with hammer and block made space under the arches without much external evidence, but the rear spring perches had to come off :

The chassis is complete without the perch, removing it doesn't introduce weakness. Cleaning off the underseal reveals the spot welds, they're easily drilled out.

If the chassis doesn't need a spring perch, neither does the trailing arm. Spot welds not so easily found on this part, holes needed welding up afterward.

Also visible in this pic is the replacement, shorter arb drop link. It's for the Z3, pushes on/bolts in. And doesn't foul the body.
Here are the results, this is max droop :

And max bump. The springs have been removed and the car is being supported by the bumpstop. Wheels rotate freely.

The rear springs (225 lb/in) have a static sag of around 2", so I'm looking to run about 50mm lower than the max droop picture.
Bloody thing will probably be too low now...