At the weekend I finally identified and cured the problem I was having with my power steering. Big thanks to Kapteenikosmos for the info from his re-build.
I went in from the top end without removing the steering box from the car and without any special tools, and I found you can do quite a lot that way, so I'll post here how it went for anyone who wants to try anything similar.
One useful thing to know is that you can easily change the input shaft seal (and the bearing if you need to) without removing the box.
You can also easily remove and replace the upper reaction spring and ring and, although I didn't need to go that far, I am 99% sure you can also remove and replace the lower reaction spring as well. That means you could probably install a set of "firm feel" cop car reaction springs (chrysler part number 3643169) in the space of an afternoon. They seem to be available for about $45 a pair.
So - the first thing I did was remove the steering column as a unit. It was a surprisingly quick job but would be more complicated if you have column gear-shift. Three bolts at the floor plate, two nuts under the dash, knock the roll pin out of the steering coupling and unplug three wiring connectors under the dash. Then it pretty much lifts out - took me about 30 mins and some of that was removing the kick trim to get the carpet peeled back.
The next thing is to remove the valve body from the top of the box (2 x bolts and two hoses) and pull out the spring and pivot lever (it just pulls off). Then I removed the top end cover by unscrewing the big castle collar nut. Apparently these vary a lot in how tight they are. The manual lists a whopping 110 - 200 foot pounds !! After a few sharp blows with a hammer and drift mine spun off easily. I can't see how this would be much easier with the box on the bench - the chassis holds it as firm as any vise could.
So here are photos of the box with valve body and top cover removed (I'll post other pictures in successive posts so they stay with the relevant text):