Here's a line drawing of the FC chassis & subframe
In some UK LR Mag a long time ago they had a story about the vehicle you describe - As far as I recall there wasn't anything too special about it. Apart from flipping the axles over the springs, increased track and that large load area, it was a standard late Series II (Probably on the 1 ton chassis) - sort of a copy of a Dodge Powerwagon for the oilfields.
The rear end of the subframe on the FC is very similar to what most everyone with a utility makes when they swap a tub for a tray on many makes of vehicles with a chassis. Without being able to find the article on the web, I'd suggest looking at a tray off a 2 ton truck as a template.
Extra: I was looking at the LR Series Page on Wikipedia, and it's fair to say that the yellow recovery version FC has the same axles as the Power Wagon copy. How about cutting/extending a 101FC ( See Mr J's post about what the two vehicles are) and put a normal cab on it?

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