can't help with any of the technicalities on this, way over my head, but a couple of observations from reading through everything.
1.HJ60s are available with FF rear axles. not all have them and the spring rate might be a bit stiff. Andy Lomas would be your man
2. I presume this is for expedition use. Almost 100% of people I have talked to who have tried trailers for expedition use reckon they are a nightmare. I was looking at using one so asked a lot of people. They take terrific abuse and either a.) fall to bits b)smash everything in them to bits c)smash the towing vehicle to bits, notably the rear cross member. It also significantly increases the demands on transmission, engine and cooling. If you are keeping the NATO style coupling my experience is it will bang and crash like a mad thing and drive you nuts, as well as putting terrific loading on the rear cross member.
3. In sand, (and most other off road situations actually) skinny is best. A tall narrow tyre that airs down well. On the main vehicle it enables the tyre to bite down for traction while the frontal area is less, leading to less build up of debris in front of the wheel, halting forward motion. There are a few situations, like in a bog or on an icelandic glacier where fatter is best but generally skinnny for go, fat for show. Thats why the traditional off road tyre of choice is the very skinny 7.50x16. Look at the tyres used by desert racers for example, and serious expeditioners.
On a trailer a fat tyre will be like towing an anchor. the larger frontal area of the tyre massively increases the drag. You gain more floatation form airing your tyres down without the gains in frontal area. fat tyres on a trailer in most situations will be a nightmare.
4. On expeditions, reliability is everything. The part that will fail is usually the aftermarket one or the modified one. As standard as possible is generally best. Those spacers I have seen that you posted elsewhere will almost certainly crack under the strain of expedition use. Things have a habit of rattling loose after repeated pounding off road so having them attached by nuts you can't easilly check sounds like a recipe for disaster. Personally I would hate the idea of using spacers even for road use.
My advice would be try a standard trailer first before spending a fortune in time and money on mods when you may find the whole idea isn't really for you anyway. I've spent a lot of time towing military trailers, and I wouldn't like to put one behind my landcruiser on an expedition.