Unimog Club UK

Mongol
If your going to mount the wheel there I would use as big a surface mount as possible use something like panel fix,  bolt it obviously and the also make a safety strap fixed to a seperate area in case of failure.
Use a wheel from a disc brake 406 as they are lighter than the late 1300l wheel as long as you don't have flat face wheel nuts.
Also have you checked your hubs for play ? Just in the picture the rear wheels look splayed especially the left wheel
Moose