One trick you can use to assist cooling on hot hot days is to put your inside fan on full blast, and maximum heat output into the cab, but on defrost with the windows wide open.
The interior radiator is often overlooked as a cooling source, and when heavily loaded and pulling a grade, take advantage of everything you have to cool that engine down if it starts getting hot.
A higher thermostat will only keep the water inside the engine until the engine is hotter. The thermostat opens to allow water to flow out of the engine when it gets to the correct temperature of the thermostat. Once it reaches that, it just stays fully open.