I don't know much, but here are a few thoughts...
While I've had a lot of trouble keeping the 483 in my 68 Colony Park cool. The 445 in my 69 Cougar Eliminator runs like a cucumber. I downsized the bypass to 5/16. Used a new ACP 3 core A/C style radiator. 7 blade clutch fan with a shroud. Run a 180 thermostat.
Using a heater control valve to stop the return of hot water back into engine helps. If you aren't running a heater core, never loop the heater ports with hose. The heater in the Cougar has constant flow, yet it runs cool enough not to bother with blocking flow. I've found with the CP 483 blocking off heater core flow will make a 10-15 degree difference at highway speed. Blocking that path of heater core will force the a higher percentage of hot coolant to go through the radiator versus being returned hot into the engine. The same idea applies to the downsizing the bypass hose between the water pump and intake. That 5/8 port continually puts hot water back into the engine. You should have some level of flow recirculation to level out temps. Otherwise, when the thermostat is closed the coolant would be too stagnant allowing hot spots. 5/16 seems like a good balance for cars like ours. Another thing is drilling a 1/8" hole at 12 o'clock in the thermostat. It helps to purge air during initial fill. As long as the bypass isn't blocked, I don't see much other purpose for drilling a hole. I've seen blocked bypass on intakes. It seems you'd need to drill a bigger hole, like 5/16 in the thermostat, or just run a restrictor in place of the thermostat.
Get yourself some women's leggings, like pantyhose. The other reason for this is to make a sock that might catch any debris that could clog up a core tube. In a freshly built engine, I was surprised how much stuff got caught in there. pieces of RTV, rust flakes, etc. The legging sock will go into the upper radiator port, probably 10-12 inches and then roll back over the spout where the upper hose goes on. The hose will go over the layer of fabric and get clamped in place. It's a temporary thing to do because I found even when the clamp is very tight, it still likes to very slowly weep.
When the shroud is custom or factory offering, you want the fan engaged 1/3rd in, 2/3rd out. A fan fully in the shroud will spin the air around and move less through the radiator. A lot of the air moved by the fan comes off radially. The 2/3rd out will allow the air to sling off the fan and pull more through the radiator.
