To catch up on my build progress slightly (as I find more pictures on my computer):
To mount the gearbox I chocked the gearbox into position and measured the angle between the drive flange and the differential - about 0.5 degrees vertical and bang on horizontal the first try! I then welded a crush tube into a tube cut to fit into the drive tunnel, screwed it onto the bottom of the gearbox and tacked it in place. As there was a screwed in adapter that stuck out from the bottom of the Jag gearbox there was much room for a soft mount - here it is with the tube tacked in and a shim:
To fix this I removed the adapter and cut off the bottom so that it was flush with the bottom of the gearbox. I then cleaned the threads and screwed it back in with a healthy dollop of high strength Loctite. I then cast a Shore 70 polyurethane block and drilled a hole down the middle for the bolt. The bolt passed up through the crush tube, through the poly block and into the gearbox with enough space for a locking nut on top - result!
I have some doubts whether the poly is hard enough, and I wondered whether I should have bonded some metal plates onto the surfaces, however I reckon it is good enough for now.