That link that Sam put up is what you need. The first picture.
I have an old Unimog 404 and I've just fitted a dropside (pick-up) rear body to it. It already had the 3 point mounting.
The centre cross member is fixed to the chassis, then the front and rear cross members can rock left to right, like a see saw (a stiff see saw!). This does of course make the floor quite a bit higher than the vehicles chassis.
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