Coolant loss mystery - 1967 C10 283

[mr.precision]

My factory stock 1967 shortbed is losing coolant somewhere. It has the original 283 2V motor, which runs well and is in the proper state of tune.
The radiator is a 2 row job, which may not be correct for the truck.
Temp gauge sits in the left portion of the C-H sweep. I have measured with a mechanical and this corresponds to 180 deg F. Even on a hot day in Arizona, it doesn't move higher unless the level goes way down.
Radiator level will drop 2-3" even after a 30 minute drive. There's no evidence of leakage or exhaust contamination.
My thoughts are that the coolant is escaping the overflow vent while driving.
Maybe a four row radiator is the solution. Any thoughts?