Coolant level in the radiator should be about one inch below the top. Anything over that will go out the overflow. Up until 52, the cap was zero pressure. 52 had a 4 psi cap, which is safe to run on yours, just don't use one with a higher pressure, you can blow out either your radiator or heater core.
If you're only leaking out the overflow, stop-leak won't help. You're either overfilling or the engine is running too hot.
