I'm not familiar with the set-up on a montego but if you can get your hand on the radiator ,AFTER it's warmed up , shut it off and feel the radiator from top to bottom and if the bottom portin is noticably cooler then either your t-stat is stuck closed (generally that will cause boiling) or the rad. is sludged up. The other fancier and safer way is to get an infrared thermometer and check at different spots with that. Also can compare upper and lower hose temps after the gauge is at normal temp .
The impeller is on the right side of the engine (when facing the vehicle) attached to the upper cam shaft.
To replace you'll need to remive the air breather and all hoses. There are 9 bolts (8mm) holding it on. Its fairly easy to replace with patience.