Radiator overflow tank

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I recently bought a '69 with a 350/400 combo. The PO had installed twin
electric fans on a new Heavy Duty 4 core radiator and also installed a
overflow tank. My question is does the overflow tank have to be at the
same level as the radiator cap to allow the fluid to siphon back into
the radiator? Right now it is below the cap and the overflow tank stays
full but the fluid level in the radiator is rather low. Thanks.