[Brian]

If you slow the flow down of the coolant as it passes through the radiator, you are likewise slowing down the flow of coolant through the engine, which means it is going to pick up more heat, consequently will be at a higher temp as it enters the top of the radiator. catch 22.
The earliest engines [including model T's], relied on the fact that hot water rises, therefore a natural tendency of coolant in an engine cooling system is for the coolant, as it picks up heat from the engine, to rise which automatically draws in cooler water from bottom of radiator. [thermo-syphon]. This is why the top of the radiator is at the highest point in the system, certainly quite a bit higher than top of cyl head.
With model A's and the first V8's, it was discovered that fitting a water pump at front and top of cylinder head[s], this natural effect could be helped somewhat by creating a positive flow , thereby improving the cooling effect of the radiator. As the owner of a 1935 Ford, with head mounted pumps, I know only too well of a major drawback in that system; at high engine speeds, the pumps actually push the coolant so fast, that some of the coolant goes outta the overflow, this lowers the level of coolant, the longer it goes on, the more coolant is lost, the hotter the engine runs....
If one fits the original, hose mounted thermostats into one of these early 221's, one of the side effects of this is that the stats, by reason of their smaller openings, will slow down this flow somewhat, thereby reducing the coolant lose outta the overflow. Fitting restrictive washers in the top hoses would have the same effect.
Starting 1937, Ford relocated the pumps into the block, where they now draw cooler coolant from bottom of radiator to circulate throughout the engine. This is a further improvement, as now, at high engine speeds, there is less tendency for the coolant to be pumped straight outta the overflow.
Such was the improvement, that, using 1939 Deluxe as an example, the top of the radiator is considerably lower in relation to the engine, also, the cooling fan is running at engine RPM's, being mounted directly on crankshaft pulley.