Got the job done.
Phase 3 took a while because I had to weld some plates to the chassis, so that I have captive nuts to bolt to. Obviously this is not a job you want to mess up

Unfortunately my diff mounts did not arrived in time, so I will have to take the diff back off again at some point.
I was a little worried that the brace might not make that much difference and that I was putting in a lot of effort for little change. However this is not the case! Just rolling the car over the bump at the end of our drive feels totally different, much tighter. I have not been for a long drive yet because I need the alignment doing, but I think it will be like a differnt car.
Next job -- Adjustable top arms on the rear. Going to use cam bolts, like the ones on the lower arms.
Si.