A further update from George on his Jimny to Willys Jeep conversion.
The "genuine" Willy's Jeep tub has turned up (they are still being manufactured by one of the original licensed Jeep manufacturers in Belgium so its a "New Old" tub)
As you can see in the picture below the chassis is longer than the tub. In the UK the law is pretty flexible around what you can do to a vehicle without having to jump through paperwork and engineering hoops. However, the "certification" and identity of a vehicle is considered to remain with the chassis, therefore chassis modifications (cutting etc.) are more difficult to get approved. Therefore the easiest option is to extend the body.