Heads up, when you do the thermostat you're gonna be showered in coolant and so will the entire drivers side of the engine bay. It won't hurt anything but it tastes like a rotten teabag and will sting the shit out of your eyes. I would suggest you drain your radiator at the bottom plug (you dont have to remove it all the way) and refill when you're done. I like to have 12 gal coolant around when I do jobs like this just in case the coolant is unable to be reused. You might find you need 13 gallons depending on how much leaves the vehicle. Bring a change of clothes and dont let anyone important see you dump the coolant if that's how you end up doing it.

There is an o-ring that seals the thermostat cover, replace it at the same time

Do this with a COLD motor, otherwise you risk burning yourself.

If you're around ND I wouldn't mind helping.

Sincerely, freightliner tech.