This is my take, for what it’s worth, on the OP. Many (me included) go the ‘fabricating body’ route to enable more control over the weight and dimensions, and to have the flexibility to accommodate their exact requirements. There’s nothing wrong with a ‘comms cabin’ conversion, but you are kind of stuck with the dimensions and weight. Forget getting under 7.5 tonnes if you go this way.
Owners fabricating their own body don’t always take the torsion free sub-frame route, but instead build directly onto the standard flat-back bed. The standard LD T244 bed is itself very rigid and is attached to the chassis with a combination of rubber mounts and bolts (at 8 points) and so does allow a bit of chassis twist as a standard feature. A ‘comms cabin’ is very heavily built and any tiny amount of flex that does reach it through the standard bed will not trouble it at all – under any circumstances. They are engineered to take a hammering.
Likewise, a self-made box, if very strong, should not have any difficulty in coping with the torsional stresses it receives. A major caveat here is that for extreme / prolonged / unsympathetic off road hammerings, then a self-made box on the standard bed - if not as strong as a ‘comms-cabin’ – will probably eventually fail. Don’t forget that as well as torsional stress that it will also be subject to shock / vibration / and a general shaking about passed up through the suspension.
For mainly road and sympathetic off-road use then a self built box on a standard bed should last the course. Indeed, I’ve put my trust and money into this solution myself. Login - Trip Truck
for more info.
For extreme, prolonged, or thrashing-it-off-road use, then it makes sense to use the ‘comms cabin’ approach, or to rip off the standard bed and build some sort of torsion free rig to support a self made box.
It is seemingly a bit of a black art and there is no one foolproof solution, and probably no-one out there that can give an absolute definitive answer. Whichever way you go there will be compromises.
One thing for sure is that if you build a box directly
onto a truck’s chassis rails then it / the chassis / mounting points will very probably fail in even mild overlanding conditions.