Really feels like I've made some progress. I finished machining the spacers for the swingarm and made a couple of reducers to sleeve the swingarm pivot holes down to the 15mm axle I'm using.
With the reducers in place I put the swingarm in the frame.
With the swingarm in place I turned my attention to the rear brake. I have a modified TZR rear disk bracket but it won't fit the new 'arm. I also have a Harrison calliper that I bought for my FJ but never fitted. So I got out a piece of 10mm alloy sheet I had lying around and after a day of filing ended up with this.
I'm still undecided about this mounting, I might change the shape and I'm going to try hanging the calliper below the swingarm.