A dinged up or bent dowel pin can also cause no end of trouble trying to set the head down without crushing any of the ferrules. (don't ask me how I know, I'm trying to forget)

Probably the best way to check the performance of the water pump etc is to use an infrared type thermometer to check temperatures of the various water passages and block. If you don't have one of those, fill the radiator too full and watch the coolant for movement as it warms up to operating temperature.

The D318 water pump came along before the plastic impeller was thought of by some penny pinching fool, so if it turns freely and doesn't leak, you don't have much to worry about.

Thermostats or "regulators" as CAT calls them are worth checking in a hot water bath. The original style with the retainer can be hard to remove. I removed the top radiator hose and the top of the thermostat housing, made sure the coolant level was up to the level of the thermostats and heated the lower housing with a torch to test if they would open.

It's best to make sure they don't stick open either since the next problem will be "slobbering" when you get the overheating cured if the engine doesn't warm up properly.