Offroad-Express

was told a triamph overdrive might fit into a series 3 landrover box. any body know if it does?

I doubt it as the Triumph overdrive unit was an extension of the gearbox and wouldn't adapt easily to the land Rover gearbox.
Land Rover overdrives tend to be additional units that bolt in where a PTO unit normally goes.

However given enough money, resources and time anything is possible.

I did some googling on this today and all the forums point to it being a rumour.