I spent a few years working in a radiator shop. Scott is right on!! A 1960's closed system with no overflow tank should not be filled to the "brim" so to speak. Most will find their own level which is an inch or so under the filler. You will get the feel after a bit and once in a while a small amount may be added.
If you keep the water right to the top some over flow will occur. I have seen instances where a closed system completely full with no expansion area in the top tank water can actually “bulge” the top tank. Normally a truck pulling a trailer and working hard. Or if you romp on your stang real good in the same situation.
The air in the top tank can compress and compensate for this sudden surge. Yes, the cap is designed to release pressure, but not quick enough in this hydraulic situation.
Never fill to the brim! Leave some air space.
Rob