I tried everything when my 1985 300D was overheating, but the symptoms were different. It would overheat at long stoplights on >90 F days, but never when driving at 35+ mph. I never took it on steep long grades in the Sierras. I tried several fan clutches, T-stats, citrus acid flushes, used radiator (flushed well), even removed the T-stat housing looking for something plugging the flow. Finally bought a new radiator when found affordable price ($130 vs $350 before) and that solved it. Tore open the old radiator and found a thin coating of "metal mud" covering the tops of most tubes. It flaked off easily, so surprised it didn't work when I ran a SS grill brush over the tops. The new radiator was also an aluminum core, said to cool better.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans