I am affraid your target millage will not be achieved with any of these engine in the old cruiser.. smaller engine have a hard time moving the brick on highway speeds
A properly built and tuned 2F can give you upto 6km.. a diesel 3B can give upto 8km.. both are proper engines for the landcruiser
If you are interested in newer generations than 1FZ-FE or 1UZ both will give you 8km max
Also do have a look at gear and axle ratios as well
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