If the thermostat is working properly, it is not the cause of the higher temperatures. Thermostats limit the MINIMUM temp. Make sure you do not have a collapsing lower hose or coolant flow restriction. When you are cruising, what is your speed? If it is lower speeds, then you might have a fan or ducting problem. Not sure if it pertains to the 340 but there was two different rotational direction water pumps and if they get mixed up, you have heating issues too.