Dear analogdad,
Hope you're well. If the oil cooler is in good order, I concur in view that the coolant/water ratio was wrong.
The in-radiator automatic transmission fluid (ATF) cooler is unlikely to have failed. It consists of a heavy-duty, double-wall copper tube. For this unit to fail, it would have to pin-hole. That's possible, if the coolant was seriously neglected for years. If so, you'd see a pinkish scum atop the water in the coolant overflow tank. Also, the ATF would be milky/cloudy, rather than a clear, rosy red.
Hope this helps.
Yours faithfully,
Spook
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