[mass A man]

My option would be to pull core out and repair it, or have it repaired by a radiator shop. I suggest not pouring in any stop leak just yet. Without seeing it myself, sounds like an easy fix where the vertical tube is soldered to the top tank and there may be where your crack is. There are other ways to pinpoint where the leak is. Over-filling, bad cap gasket, etc.. is ruled out since you say you can see where it's leaking. To solder it up yourself, you must know how to solder a radiator core. Shiny, clean is mandatory at the point of repair.