If the cooling system is top notch, the thermostat should be able regulate coolant temperature pretty close to the chosen value.
The 261 is a newer engine and 'probably' designed to run at the higher temp...180! Parade duty can be tough on the rigs cooling system unless it is clean and working well.
The fact that the truck has a heater should have no bearing on thermostat selection.
If the tubes in the radiator are partially plugged and the block is cruddy from rust and such, be prepared to 'steam' your way through the parade route.
Stuart
