IMHO the 300TDi engine in the Discovery is waste of space. The high mounted water pump and the Siamesed centre exhaust ports virtually guarantee head gasket failure if one hooks 3.5 tons onto the tow ball. Some way of getting the header tank higher needs to be looked at. A V engine or even half a V like the 1970s Vauxhall Victor would be much better because the cylinder heads would then be lower. Another way might be to fit a low mounted water pump outboard of the block. I loathe my Discos for their poor cooling system design. Curiously when I was running a Triumph GT6 many years ago there were no problems at all with steam trapping or air venting as even with a low bonnet line the radiator was higher than the engine as it should be. Caveat Emptor and good luck as you will need it!