Overheating issues
...a bunch of work that won't matter, measure your pulleys.
My stock 67 Dart has no fan shroud and the fan sits 3-4 inches from the radiator (stock distance) and It can idle all day long without issue on the hottest day.
My 67 273 has 6 1/4 Dia crank pulley and a 6 1/2 Dia water pump pulley
IMHO Regardless of a thermostats flow it will only allow enough water to flow to maintain the water temperature it is designed for. It opens and closes depending on temp and that will increase or decrease the flow.
My bet is you have some other issue, trapped air, bad water pump, blocked passages, clogged rad cores, too small radiator core, pulley speeds etc.
Also some details about the engine, rad, year, modifications, etc would be very helpful. Are you running a 440 with a slant 6 radiator?
