OK, job done.
The reason for not simply swapping the box for a supposedly reconditioned unit was that I kept reading reviews which suggested that many of these units are just used units that are cleaned and painted. I wanted a box that felt new, or as close as I could get.
Turns out there were plenty of guides available on the web. I used
this one.The hardest part was reinstalling the ball-bearings. It took me ten or so goes to get my head around it. Also, make absolutely sure the steering worm and valve body are aligned with each other. The slop in my box meant the two had separated - difficult to explain, but there's a pin that keeps everything in register, and it had skipped out of the notch in the valve body and re-seated (bound?) itself off-centre. I think this may have contributed to my steering not centering correctly.
I also fitted a new rag joint to the stock steering column. I had done the Jeep XJ column mod, but I felt that it didn't suit the character of the car. The new rag joint made a world of difference. The steering is now isolated, but there is no central play. You can palm the wheel with very little effort, and it rolls back to center with that big, lazy GM cop-cruiser vibe. I think it feels like a big Buick should.
Whilst I was oily, I fitted new spark plugs and leads. I cracked a plug (cylinder 2) and it took me three days to source the misfire. That was annoying.
Also cleaned the MAF. Tried the hockey-puck intake thing but couldn't get on with it either, so I put the intake home plate / resonator back on.
Anyway, the car now steers and goes beautifully. Really worth the effort.