I wouldn't be worrying too much about fuel economy. If it is a diesel it will do about the same regardless of what configuration it is in. I have come the conclusion that the aerodynamics (and speed) make far more difference than anything else.
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^^ 110 hardtop with a disco transfer box and disco 200tdi, running 265/75 Bfg AT
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^^ 90 hardtop, TD5, standard drivetrain, running 265/75 Bfg AT (ignore the best MPG that is an error)
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^^ 88 series 3 softop, 2.25 diesel with OD, running 7.50 Michelin XZL
As you can see there is not that much difference between any of them and they are all averaged over a long time (especially the 110). With an old land rover there is much more important things to worry about than MPG ad if you are worried about MPG the easiest way to improve it is alter your driving style rather than modify the vehicle.
The 110 above (1989) was my daily for 13 years before the engine fire, then the td5 above became my daily and once rebuilt the 110 will be back as a daily drive, I am doing about 30k a year so don't listen to anyone who says they cannot be a perfectly usable daily drive.