dhls, thermostats set only the MINIMUM operating temperature (opening point of thermostat).
Replacing with higher minimum temp thermostats will not make the engine get hot if it already does not reach the 80 to 85 Celsius temp thermostat.
If this engine/application uses some type of de-aeration or air eliminator hose between the engine and the radiator, someone may have replaced a restriction fitting with a standard wide open fitting sometime in the past. Use of a standard type fitting in the air eliminator circuit can caused cold operating temps in very frigid weather because the air eliminator hose bypasses the thermostats (and reduces the flow through the heater circuits).
Another thing to consider is "just how danged cold is it" where the truck is being used" and how much actual work is being done by the engine.
If it is really, really cold and the truck is mostly just running at idle, the heater core itself may just be "over cooling" the engine. The heater core is usually plumbed into the "radiator bypass circuit", so very cold cab temps can actually over cool the engine unless the engine is working hard.

you can try a hotter thermostat but i'd check a couple things first

- make sure the bypass circuit on the cooling system isnt blocked, if its clogged up and not circulating properly it will make the engine take a long time to get hot.
- does it have an add-on heater system on it? this will make it take longer to heat up also, more coolant to deal with and if fan is running on heater that will hinder the coolant heating up too. (most of the time these heater units are tied into the bypass circuit)

:o)