[Chrysler300] Transmission cooling line failure inside radiator

I have transmission fluid contaminating the cooling system, and water contaminating the fluid in the transmission! Seeking Advice. How to decontaminate transmission and entire cooling system.

Description of problem:   After manually refilling the Type F fluid (about 10 quarts) in the newly rebuilt Torqueflite in my 1957 300C, we came back the next morning to find a big pool of water and transmission fluid under the car! They removed the transmission pan, which let out a considerable amount of water and oil mix. The Torque converter was completely drained. There were remnants of pink oil visible on the cooling passages when the radiator cap was removed. The radiator was drained of the remaining oil and water mixture.

It was quickly concluded that the tubing for transmission fluid at the bottom of the radiator had ruptured internally while the engine was being run to perform the fluid fill process, pumping high-pressure pink fluid into the radiator water while running. But after shutting down for the night, the heavier water pushed back through the (now unpressurized) ruptured oil cooler line inside the radiator, which eventually pushed oil and water out some overflow location near the back end of the transmission.

The transmission was being run in Neutral, Drive and Reverse during the fluid fill process, so we were unwittingly getting some water back into the ruptured oil line as soon as the engine was shut down and the transmission coolant line pressure went to zero. Then as soon as the engine was restarted, we were pumping some oil into the radiator and also pumping some oil/water mix through the transmission.

The entire cooling system (radiator, all hoses, all water passages in the engine block and heads, etc.) is now contaminated with at least some amount of transmission fluid, so I am seeking advice on how to best remove that oily residue. We plan to blow out the transmission fluid lines at the bottom of the radiator to remove as much residual fluid as we can, and then just cap off the inlet and outlet lines, so there will no longer be the original oil/water heat exchanger functionality. (That transmission fluid cooing aspect will be addressed separately)

Our tentative plan is to refill the radiator with a mixture of water and a water-based degreaser like Simple Green, or Purple Power, then run the engine until it's good and warm, then drain out all the coolant with solvent and oily contaminant. We will probably do the solvent flush routine until we see no further trace evidence of transmission oil in the water coolant -- at least twice -- and then refill with distilled water and a water-based rust remover called Evapo-Rust to remove some of the rust that is most certainly all through the internal coolant passages after the vehicle sat idle for 17 years.   I was planning to run that rust-remover coolant combination for several hours, or at least half a dozen hot/cooldown cycles before going with a more permanent water and antifreeze mixture for everyday driving.

What to you guys think of that plan to remove residual tranny oil from the coolant system?

We plan to connect the transmission cooling lines to an external auxiliary oil-to-air transmission cooler that will be located in some open space roughly in front of the bottom of the radiator wherever it looks like there might be some air circulation path.   I was not planning to have any fan-forced airflow device, relying just on convective cooling from a fairly generous finned heat exchanger in whet will likely be a rather turbulent air flow region (but I have NO way of knowing that!) I don't know what temperature the transmission fluid would like to be when it goes back into the tranny after the cooling process, although I suspect it would be somewhat higher than the temperature in the radiator, around 190 F as governed by the thermostat.   I will have no reasonable way to tell what the oil temp is when exiting the auxiliary heat exchanger, so I won't know if the heat exchanger is doing a good enough job. I know the air temperature will usually be considerably cooler than the 190-degree water in the radiator, but I also know that the heat exchange coefficient of the oil-to-air cooler is much lower than the HX coefficient of the original oil/water cooler system.

What do you think of the oil-to-air auxiliary heat exchanger idea to replace the original oil-to-water system? Any thoughts or advice are welcomed!