Frank, I'm putting a 406 powered steering box on mine, but can't advise as its still sitting on the ground next to my Mog...
If you want to go ram assist, in the UK try steerforth for rams and hydo bits. They'll do you whatever cylinder you need.
As has been mentioned, the ram assist setup used on old series landrovers is probably the easiest way. That kit uses a valve from the guys in Spain, above (TDZ). It works well, but I had a hell of a time trying to get them to sell me anything a few years back. The webiste looks better now and they have an order form, so maybe they've bucked up a bit.
See here for an example of the kit: (select 'power steering on the left)
Heystee Automotive bv, Special parts for Series Land Rovers
Cheers, Al.
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