To answer the original question, yes you can find a 120 degree T stat for marine uses. Not sure if it fits our trucks.
I'd run the truck without one instead.
I run my truck with a 160 tstat. I also use a "water wetting" agent in the coolant. Truck runs near 175. Reason I do this is to keep the engine cooler in summertime traffic.
I also uses a manual valve on my heater hose to keep the cab cooler.
