Think long and hard before doing it. I have had a 90 witha 6.5 fitted, Ashcroft heavy duty ZF auto, this was a sweet truck, only problem, keeping it cool, had special big radiator made, but fans going nearly constantly, and hard towing, you really had to watch the gauge.
The second was a disco, which eventtually ripped the arse out of the transfer box, when pulling away with a loaded trailer, the torque is to much for standard landrover gearboxes diffs, and axles.
The 90 eventually would cause the HD auto to slip the clutches in top, but on the plus side, pulling away from lights, with a muppet in a hot hatch was always fun.
Oh and its dam heavy engine.