The "freeze" plugs are merely casting plugs for cleaning out the sand from the casting processes. These 215 blocks will crack along the upper drivers side along parlell to the top of the block usually near the front of the block. They will crack there before they ever press out a "freeze" plug. Use 50/50 coolant. Gene has recommended this many times.
As to the radiator, like Gene, I recommend taking the radiator out. The hardest part is the rods from the cow to the radiator support. When you have it out use an air gun on your compressor and a rag at the bottom outlet. It will come out by the fan if you loosen the belt. Use a garden hose to keep water flowing. Blow up (reverse) from the bottom outlet and let the water gush out the top outlet and by garden hose.
Keep doing this until the water runs clear. Fill the radiator to the top while holding you hands over the bottom and top outlets. When full release the bottom hand and notice the flow of water. It should shoot up about 5 inches and go down without much lagging water. Listen for any sucking sounds as the water empties out the flues. Such sounds indicate flues that are partially stopped up. If so and you didn't clear enough flues the core may need to boiled and rodded out. Not a difficult job if you are good at soldering and know how to take the top tank off and put it back on. (That entails blowing off the old solder, cleaning the tank and soldering it back on.) You need an acyetlene torch for this and a bench grinder with a brush on it. Some cut hydrocloric acid will help. "Cut" it with some pieces of old jar lids. (More on that procedure upon request. Best left to the radiator shop.)
I don't see any point in removing the "freeze plugs." Won't hurt but the nuisence is not worth any gain you may realize.
Same with the water pump (circulator). You need it in place to reverse flush the block. Again, using the air compressor, fill the block with water and blow back out through the block petcock. Do this until the water runs clear. If the block is so stopped up with rust that this doesn't work then it needs to be tanked. (major operation and work there) (take the thermostat out whilst doing this.
Check the fins in the radiator while you have it out. Blow out and straighten and bent fins. Get the trash out so air can pass through.
If you're loosing water (coolant) then check for a blown head gasket or cracked head or block. Easy, with the thermostat removed, fill the block with water up to the upper neck (no hose), trip the fan belt, and crank the engine. The water at the upper level of the neck should stand still when cranking the engine. If you see bubbles or water gush up then you have a crack or bad gasket.
As Gene said, the 41 had a good cooling system. And it was good enough to run cool with about a quater of the flues stopped up if every thing else was okay.
Note: The ambient tempature shouldn't be a concern. The thermostat will bring the coolant up to the its rating degrees of temperature. The rest of the cooling is up to the radiator.
If the tempreture goes into the read. Open the hood and turn on the coolant flow to the cabin heater. That is merely another radiator and will help some if you can stand the heat. Not so if its stopped up too.
Good luck,
Charlie
BTW: I could have told you to do a search owing to the fact that I've posted this advice many times. But, nothing beats a direct answer to your post.
BTW2: I doubt you have a true "honeycomb" radiator. Those had the zig-zaging flues. The 41 Chevrolets had straight flues. I think those stinkin Chrysler product may have had some "honeycomb" radiators withthe zig-zag flues. They can't be rodded. A McCord core was the usual fix there.
TRW3: Sorry this was so long. I get carried away sometimes.
