The following is from the 1970 Challenger Factory Service Manual (FSM) concerning the thermostat.
THERMOSTAT
The thermostat is actuated by a pellet containing a
copper-impregnated wax, as shown in (Fig. 5). As the temperature of the pellet increases, the wax expands and opens the valve.
A 190* thermostat is standard equipment.*
The use of 160° thermostat or alcohol type anti-freeze is not recommended.
If the thermostat does not close completely when cold, the engine will warm up slowly or not at all,
and heater performance will also be impaired. Poor
heater performance may also be due to valve opening
at too low a temperature. Too high a valve opening
temperature or a valve that will not open can
cause overheating.
*318-383-2BBL have a 195* thermostat.