If an overflow container is mounted near the top of the radiator, and the end of the tube is below the level of the liquid in it, the radiator will refill itself from siphon action as the engine cools. That's how modern coolant reservoirs work, and you can scavenge one from a salvage yard that fits the space you have available.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!