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One question... Why? Unless you have a constant boil-over situation which is a bigger issue I wouldn't mess with it unless you are going to replace the radiator that doesn't have the tube.
If you use a recovery cap and the overflow can the Radiator will stay full to the top and wont overflow onto your garage floor. I usually add them to my cars.
The probably easiest possibility is an after market plastic tank mounted on the passenger side inner fender apron. This is a generic product. the hose to the tank is attached to the overflow tube usually by cutting it before it goes down the side of the tank core.
That has never happened to me and 1/2-1" from the lip is the normal allowing natural expansion in the system.
I think I’m over filling it. I filled it to the top and after a 200 mile jaunt it was down about an inch from the top.
That's exactly what the system likes. Let it be. Some of the newer cars have high spots that don't like to fill with coolant and often have bleeders in the system to remedy. Not with our old simple systems The only air is in the top of the radiator and in old days was called the expansion tank.
When you look inside the radiator neck you should see a baffle with a hole right below the opening. The fill level is to the baffle and visually helped by the hole in the baffle to check the level. the fill is not supposed to be to the brim. When you fill to the very top of the neck you will expel coolant when you next stop because you have overfilled the radiator.
Just over the baffle usually works fine too. It's kind of interesting that the system will spit out any overfill until it reaches it's preferred level as Zeroroadkill discovered. Oh and a proper cap is important too.
I’m positive my cap is not the original (no psi noted) and I noticed when I did overfill it coolant was around the tank top below the cap as well as out of the overflow tube. The manual on page 338 says “cooling system is pressurized to 6 1/4 to 7 3/4 pounds on all cars”(?) and on page 98 it says “the cooling system of the passenger car is pressurized at approximately 13 psi”. CASCO & NPD show 13 lbs replacement caps for the’55.
