It is much easier to route your 3" exhaust if you stay outside the frame an let it end just before the rear right wheel. One very open muffler will work well together with the turbo.
Shatbox HDJ is LHD (like 99% of cars in Spain). If the bends are really different between early and late models
(what I doubt) , it can be solved with a different peace of rubber hose or with the soldering iron.
In my opinion the 80 is one of the best 4x4, especially as a turbodiesel. The idea was to add to the original body a roof top tent and a spare tire to the back, keep with this measures the original with/length/hight on a virtual car.
Keep this measures to plan a box or cabin. That is what you...