Hi cody,
bad thermostat or wrong rotation water pump. You should be able to detect a wrong water pump with the casting #. The main problem is if the waterpump turns the wrong way the coolant will flow through the engine in the wrong direction, that's what might be causing the overheating. It should flow from the cooler parts of the engine (block) to the hot parts (heads). That way the heads get coolish water. If coolant flow is reversed it flows through the heads first and then goes into the block hot. That heats the block much more than usual and causes a whole pile of problems, like too hot oil and stuff. So if the thermostat proves OK, check the rotation!
BTW: slight overhating isn't much of an issue for a new engine, because friction is much higher. If it's running just a little on the hot side (with correct rotation and thermostat OK) just pop in a slightly cooler thermostat for the break-in period.
Hope I could help,
Max