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1978 E250 C-6 transmission rebuild normal verses synthetic transmission fluid

Hello, my excellent transmission guy says that if I use synthetic verses normal trans fluid I will not need an additional transmission cooler. Is the extra cost worth it? I would like your opinion on this matter. I will be pulling a very fully loaded van and 12 foot trailer to the mountains many times. I have a new radiator installed etc. The rebuilt motor has about 30,000 miles on it. However, my engine has a mysterious heating problem. If I get into more than 4 traffic lights and the outside temperature is above 70 degrees it wants to overheat. I have done everything imaginable. New radiator, water pump, thermostat, new fan shroud and fixed position fan (not the clutch type) no external coolant leaks of any kind. Heater core replaced also. Spring coil inside the top radiator hose so it will not collapse. Running down the road though, it runs a bit below or at normal temp wise. Very strange. If the heads have been shaved too many times or if a cylinder or two have been bored out to the max could this cause this? Engine was purchased via an auto parts store rebuild several years ago. Thanks ahead of time.

How much does a transmission cooler cost?