Re: overheating issuse
What sort of radiator, what sort of fan, what sort of shroud, what size pulleys are they (roughly; are they ford or aftermarket), what sort of water pump.....?, there are a few variables.
But, your symptoms sound like a shroud issue (at a guess anyway, without all the relevant info). A shroud not only makes the fan suck air through the radiator at low speed/idle, but it also slows the air down at high speeds on the road. If the air is going too fast (around 100.....) then it doesn't have time to absorb heat from the radiator as it rushes through the core. If you have a shroud fitted (of the correct type), then the air has more time to pick up heat and the engine runs cooler.
That's guess number one. Most people don't seem to realise that the most efficient cooling system (rad, fan, shroud, pulleys, etc) is the one that Ford designed. Obviously a strong engine will need the best that Ford offered (3 core, extra width radiator for XA onwards, flex fan, correct pulley sizes, and, most importantly, a functional thermostat !!!...).