Hi, I parted one out a few yrs ago and the cross cradle had a spot for moving the DS motor cushion forward to install a 617. there was a thread about a month back with a hymer 309d rv--the bigger bus body with a turbo 617 and an automatic. a 617 would bolt right in and your truck already has the special 33 lb flywheel--a G-wagon flywheel to go on a 617. if you are not a speed demon you could settle for 5 cyls naturally asperated--just find a rusty old heap and yank the motor--and swap in a long runner 115 5 cyl intake at the same time--and its dry aircleaner can too. If you were in the states you could (maybe) find an auxillary transmission -browneee-- that you put in the driveline that wold give you OD as well as low for each gear forward or reverse, they are tough to find nowdays, but they make small engines--the little engine that could!
if you try to mod your 616 it will most likely cut the life --especially hauling this bulk around the guys that do it take a 617 turbo exhaust and lobb off the last runner and weld it up with a nickel welding rod. Your special long runner 616 intake is a redesigned om621 intake with the inlet at the front end--and has a throttle flap if i remember correctly--you would just remove that and run the air straight in.