Let me add the following:
Radiators have 1/2" risers that run in wall so I can't change them.
All zones re-piped with 1/2" PEX in a series loop.
First floor radiators call for about 40,000 BTUs while the 2nd floor is about 15,000 BTUs. Note: I calculated based on an old EDR chart I found from the 40's.