Freight trucks, and the like, get away with hard mounting the boxes on the rear because driving on sealed roads does not cause the chassis to flex excessively. A Canter intended for off road use is a totally different story and cannot realistically be done in the same manner, as the environment in which it will operate is quite different.
I do not profess to be an expert in regard to mounting systems for camper bodies but using some basic logic I believe that allowing unrestrained flex of the chassis is as bad, if not worse, than hard mounting.
When a camper body is hard mounted it tends to focus the stresses and, in a Canter with a stepped frame, this is normally just in front of the step. This will naturally increase the potential of cracks in this area if the chassis is twisted a lot, as it would do in more harsh off road scenarios where wheel articulation is greater. Conversely, if the chassis is allowed to freely flex, this will place additional stresses on the chassis welds and rivets, which can also be detrimental.
In my opinion, for what it's worth, the chassis should be allowed to flex independently of the camper body but it should also be constrained, so it cannot flex too much.
This is why I believe that a fully spring subframe mount is the best solution for a Canter. The springs allow for the chassis to flex independently of the subframe but at the same time, they stop excessive chassis flexing.
Changing the suspension to something that allows more wheel articulation will also reduce stresses on the chassis.