451 overheating, even when moving

That fan shroud blocks more of the radiator than it allows flow for. Also, two electric fans aren't working and you want to add a third? Why not do some actual diagnosis first? Throwing parts at something is not the way to do it. Get an IR temp gun and shoot the upper hose going into the radiator and the lower hose coming out of the radiator and see how much of a difference there. This will tell you how good the radiator is. The wider the split between those two temps, the better job the radiator is doing. Also shoot the thermostat housing and around the temp sender to assure the engine and the gauge correspond.