A pressure type radiator cap (4 lb) was first used on light trucks in 1953; and, was used on passenger cars and COE trucks from 1948. Passenger car (and maybe truck) engines might have changed to a 7 lb cap in 1955 but my books do not make it clear if this was for all engines or just V8s.
Most likely a 4 lb (or maybe even a later 7lb) radiator cap can be used on a 216 engine. In all cases (0lb. 4lb, or 7lb caps), a radiator filled to the top will overflow until the fluid (when cool) is an inch or two below the top.
cmayna,
Keep checking your radiator (when cooled) and see if the level stabilizes.
Tim
