I think you are missing the obvious. If that is the original radiator then it is 38 years old! You said the coolant was disgusting. What do you think the inside of the radiator looks like? At least I would take it to a radiator shop that has a heat gun and check the radiator from top to bottom.
Or bite the bullet, spend $300 on a new radiator and thermostat and be done with it!
Anziani
'93 320CE 205K
'95 E420 125K