I didn't read the whole thread, so apologies if you've answered this.
But what exactly is wrong with the ride? I see mention early on it's too boaty and hits the bump stops off road. So does it soak up bumps on the road too, or is it very rough.
If you are just getting lots of lean, then that probably isn't just springs. If it's ride, then springs & shocks most likely culprits.
Honestly I've never looked into it, but I assume a standard sized LR springs fit. If so, maybe see if you can blag or buy some used ones, maybe off a Disco or something. You really want to get hold of some standard OEM springs. Then it's just a matter of swapping them in and seeing if they improve things.
I'd have thought if you contact someone breaking a Disco, they might let you have all 4 springs for £30 or so.
Another thing to look at are wheels. 18" rims can make the ride less supple compared to 16" rims. Also worth checking tyre pressures too.
That said, my p38a runs on 18's, but it is on air. It rides sublimely.
Exactly where are you going to get OEM coil springs for a P38?