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. 

 George decided to extend it above the rear wheels, so the tub was cut in half.

WIth the tub cut a new insert was welded into place. This also raises the floor of the rear of the tub to clear the Jimny fuel tank. In the lower picture you can see the frame built to support the floor.

In the front, the transmission tunnel needs modification to house the Jimny gearbox/transfer box and as the Jeep fuel tank (under the LHD drivers seat) is not needed this is plated over.