Just reporting back that replacing my radiator solved the overflow problems and, along with the previous head gasket replacement, solved the black silt contamination in the coolant.
Engine easily maintains temps now, never overflow leaks, and coolant level stays constant and color is clear (I use the blue stuff, Pentosin/pentofrost NF).
Also noted that I had bits of rubber in the system before (from a thermostat gasket that got sucked into the system and chewed up, and pieces even made their way to hose connections at the radiator, especially that small one above the temp sensor at the top driver side corner, and this further created nasty blockages.
So point being, if you change your radiator, make sure all hoses are clean and clear, and if they are old, may want to just replace them as well.